GDI  If 

C6 


WORKS   TRANSLATED   BY 
WILLIAM   T.   HALL 

PUBLISHED   BY 

JOHN   WILEY  &  SONS 


P.  P.  TREADWELL'S  ANALYTICAL  CHEMISTRY 

IN  Two  VOLUMES. 
Vol.  I.   Qualitative  Analysis. 
8vo,  xiii  +  534  pages.    Cloth,  $3.00. 

Vol.  II.  Quantitative  Analysis. 

8vo,  xi  +  903  pages,  126  figures.    Cloth,  $4.00. 

.  C.W.   KOLFE.) 

H.  Claassen's  Beet-Sugar  Manufacture. 

8vo,  xv  +  343  pages.    Cloth,  $3.00  net. 

^ith  A.  A.  BLANCHARD.) 

H.  and  W.  Blitz's  Laboratory  Methods  of  Inorganic 
Chemistry. 

8vo,  xv  +  258  pages,  27  figures.    Cloth,  $3.00. 

(With  J.  W.  PHELAN.) 

H.  Blitz's  Introduction  to  Experimental  Inorganic 
Chemistry. 

12  mo,  vi  +  185  pages.    Cloth,  $1.25  net. 
(With  GEORGE  DEFREN.) 

Emil    Abderhalden's   Text-book    of    Physiological 
Chemistry. 

8vo,  xiii  +  722  pages.    Cloth,  $5.00  net. 
(With  H.  S.  WILLIAMS.) 
W.  Ottwald's  Introduction  to  Chemistry. 

Largo  12  mo.,  ix  +  368  pages,   74  figures.    Cloth, 
$1.50  net. 

(With  C.  R.  HATWARD.) 
W.  Borchers's  Metallurgy. 

8vo,  v  +  271  pages,  218  figures.    Cloth,  $3.00  net. 

A.  Classen's    Quantitative   Analysis  by  Electrol- 
ysis. 
8vo.  xhr  +  308  pages,  51  figures.    Cloth.  $2.50  net. 


QUANTITATIVE  ANALYSIS 


BY 


ELECTROLYSIS 


BY 
ALEXANDER    CLASSEN 

WITH  THE  COOPERATION  OF  H.  CLOEREN 


REVISED    ENGLISH  TRANSLATION    OF    THE 
FIFTH    GERMAN    EDITION 


BY 

WILLIAM  T.  HALL 

Associate  Professor,  Massachusetts  Institute  of  Technology 


NEW  YORK 

JOHN  WILEY  &  SONS,  INC. 

LONDON:   CHAPMAN  &   HALL,    LIMITED 


COPYRIGHT,  1913,  1919, 

BY 
WILLIAM  T.  HALL 


Stanbope  jpress 

H.GILSON   COMPANY 
BOSTON,  U.S.A. 


PREFACE. 

THE  first  edition  of  this  book  appeared  in  1882  and  contained, 
for  the  most  part,  only  those  methods  which  had  been  worked  out 
in  the  author's  laboratory.  Examples  were  also  given  of  the 
applicability  of  electrolytic  methods  in  the  analysis  of  technical 
products.  Successive  editions  contained  the  innovations  and 
improvements  that  were  made  in  the  years  1882  to  1897  until, 
in  the  fourth  edition,  a  section  was  introduced  which  contained 
theoretical  considerations  based  upon  the  then  new  theory  of 
solutions.  During  the  last  decade,  however,  the  development 
of  electrochemical  methods  not  only  on  the  practical  side  but 
also  as  a  result  of  the  development  of  physical  chemistry,  espe- 
cially electrochemistry,  has  placed  electro-analysis  upon  a  scien- 
tific foundation.  The  advances  in  both  practical  and  theoretical 
directions  have  been  so  marked  that  it  has  become  necessary  to 
revise  the  book  thoroughly  and  the  present  edition  may  be  regarded 
as  practically  a  new  book. 

Among  other  things  the  book  now  includes  many  new  rapid 
electrolytic  methods,  the  determination  and  separation  of  the 
halogens  as  well  as  the  metals  of  the  alkali  and  alkaline  earth 
groups,  and  a  special  part  concerned  with  the  analysis  of  technical 
products.  For  carrying  out  the  rapid  methods,  the  outfit  in  use 
in  the  author's  laboratory  at  Aachen  is  described.  This  was  the 
first  of  its  kind  in  Germany  and  shortly  after  its  installation  was 
used  as  a  model  for  many  other  laboratories. 

A.  CLASSEN. 


TRANSLATOR'S  PREFACE 

THE  earliest  English  edition  of  this  book  was  prepared  by 
W.  H.  Herrick  and  a  later  edition  was  by  B.  B.  Boltwood.  Owing 
to  the  rapid  progress  made  in  the  development  of  electrolytic 
methods  of  chemical  analysis  since  1882,  there  remains  little  of 
the  present  text,  which  is  exactly  like  that  of  previous  trans- 
lations. Previous  editions,  moreover,  have  followed  the  German 
text  closely.  The  present  edition,  however,  is  a  revision,  without 
further  reference  to  the  German  text,  of  the  translation  made 
six  years  ago.  Some  new  procedures  have  been  added,  the  order 
of  treatment  has  been  changed  and  the  theoretical  explanations 
modified.  It  is  more  important  to  understand  exactly  what  is 
known  to  take  place  during  electrolysis  than  it  is  to  apply  any 
particular  theory  to  the  phenomena.  On  the  other  hand,  a 
simple  application  of  the  modern  electronic  theory  seems  to 
clarify  rather  than  befog  the  vision  of  the  beginner.  An  attempt, 
therefore,  has  been  made  to  apply  this  theory  a  little  more  closely 
than  has  been  done  in  most  of  the  other  well-known  books  on 
the  subject. 

In  preparing  this  revised  text,  the  writer  wishes  to  acknowledge 
his  indebtedness  to  his  assistants,  E.  E.  Richardson  and  S.  G. 
Simpson,  who  have  read  the  proofs  and  offered  various  sug- 
gestions. 

WILLIAM  T.  HALL. 
CAMBRIDGE,  April,  1919 

vii 


TABLE   OF   CONTENTS. 

PAGE 

PART  I. — INTRODUCTION. 

Migration  of  the  Ions 13 

Resistance     16 

Electromotive  Force  or  Potential   21 

Procedure  in  Electro-Analysis     44 

Action  of  the  Current  upon  the  Electrolyte    44 

Simple  Electrolytes 45 

Complex  Electrolytes 50 

Character  of  the  Metal  Deposit  and  Duration  of  the  Electrolysis 52 

Shape  of  the  Electrodes    53 

Electro- Ana  lysis  with  Moving  Electrolytes 60 

Rapid  Electrolysis  by  Means  of  Magnetic  Stirring 73 

Electrolytic  Determination  of  a  Metal  and  Electrolytic  Separations ....  79 

Deposition  of  Metals  from  Simple  and  Complex  Electrolytes     83 

Influence  of  Temperature  on  the  Separation  of  Metals  in  Complex 

Electrolytes    93 

Non-Electrolytic  Methods  of  Electro-Chemical  Analysis 97 

Historical  101 


PART  II. — ELECTRO-ANALITICAL  DETERMINATIONS 113 

Group  I.     Metals  Electro-Negative  to  Hydrogen 116 

Copper 116 

Deposition  from  Sulphuric  Acid  Solutions      116 

Deposition  from  Nitric  Acid  Solution    124 

Deposition  from  Ammoniacal  Solution 129 

Rapid  Deposition  of  Copper 121,  123,  128 

Silver 131 

Deposition  from  Nitric  Acid  Solution    132 

Deposition  from  Ammoniacal  Solution    133 

Deposition  from  Potassium-Cyanide  Solution 133 

Rapid  Deposition  from  Cyanide  Solution   134 

Mercury 135 

Deposition  from  Nitric  Acid  Solution    135 

Rapid  Deposition  from  Nitric  Acid  Solution    136 

Deposition  from  Potassium  Cyanide  Solution    136 

Deposition  from  Sodium  Sulphide  Solution 136 

ix 


x  CONTENTS 

PAGE 

Gold     

Deposition  from  Potassium  Cyanide  Solution    . 

Rapid  Deposition  from  Potassium  Cyanide  Solution    

Deposition  from  Sodium  Sulphide  Solution 

Deposition  from  Ammonium  Thiocyanate  Solution      140 

Platinum  ... 
Rapid  Deposition  from  Sulphuric  Acid  Solution    . 

Palladium    . 

Rhodium  

Rapid  Deposition  from  Sulphuric  Acid  Solution 

Bismuth 

Antimony    153 

Procedure  for  Depositing  Antimony  from  Sodium  Sulphide  Solution  157 

Tin     158 

Deposition  from  Acid  Oxalate  Solution     159 

Rapid  Deposition  from  Ammonium  Sulphide  Solution 161 

Arsemc            162 

Tellurium    163 

Rapid  Deposition  of  Tellurium     *.  163 

Group  II.    Indium,  Cadmium  and  Zinc    1G5 

165 

Deposition  from  Alkaline  Solution 165 

Rapid  Deposition  from  Alkaline  Solution 166 

Rapid  Deposition  from  Ammoniacal  Solution    168 

Deposition  from  Acid  Solution    169 

Rapid  Deposition  from  Acetic  Acid  Solution 170,  172 

Rapid  Deposition  According  to  Sand 171 

Cadmium      174 

Deposition  from  Sulphuric  Acid  Solution    174 

Deposition  from  Alkali  Cyanide  Solution    176 

Rapid  Deposition  from  Alkali  Cyanide  Solution    177 

Deposition  from  Oxalic  Acid  Solution 178 

Indium 179 

Rapid  Deposition  from  Formic  Acid  Solution 179 

Group  III.     Iron,  Nickel  and  Cobalt 181 

Iron   ' 181 

Rapid  Deposition  from  Oxalate  Solution .184 

Nickel    185 

Deposition  from  Ammoniacal.Solution      185 

Rapid  Deposition  from  Ammoniacal  Solution      189 

Deposition  from  Oxalate  Solution 190 

Rapid  Deposition  from  Oxalate  Solution  ....  191 

Cobalt  "  191 

Group  IV.    Metals  Deposited  as  Such  on  the  Cathode  or  as  Oxide  upon 

aodi       193 

193 

Rapid  Deposition  of  Lead  Peroxide  in  Nitric  Acid  Solution . .  196 


CONTENTS  xi 

PAGE 

Manganese    '. 197 

Rapid  Deposition  as  Peroxide     200 

Deposition  from  Formic  Acid  Solution 201 

Uranium    201 

Thallium 202 

Determination  as  Oxide   202 

Chromium     204 

Oxidation  of  Chromic  Salt  to  Chromate    204 

Rapid  Oxidation  to  Chromate 205 

Determination  as  Chrome  Amalgam    205 

Molybdenum    206 

Rapid  Deposition  as  Sesquioxide    208 

Analysis  of  Molybdenite 208 

Vanadium   208 

Group  V.     Elements  Deposited  Only  as  Amalgams    209 

Aluminium    209 

Barium,  Strontium  and  Calcium 209 

Determination  of  Halogens    210 

Separation  of  the  Halogens  by  Electro-Analysis     211 

Electrolytic  Determination  of  Halogens  and  Titration  of  Cations  . .  214 

Separation  of  Alkali  and  Alkaline  Earth  Metals  from  Heavy  Metals . .  218 

Potassium,  Ammonium  (Nitrogen) 221 

Determination  of  Nitric  Acid  in  Nitrates    222 

Preparation  of  Standard  Sulphuric  Acid 224 


PART  III.    SEPARATION  OF  METALS  227 

Copper 227 

Separation  from  Silver    227 

Separation  from  Cadmium 228 

Separation  from  Mercury  and  Lead     230 

Separation  from  Arsenic     233 

Separation  from  Aluminium,  Alkaline  Earths  and  Alkalies     235 

Separation  from  Bismuth     235 

Separation  from  Chromium  and  Antimony 236 

Separation  from  Iron      237 

Separation  from  Manganese  and  Magnesium     239 

Separation  from  Nickel 240 

Analysis  of  a  Nickel  Coin 241 

Separation  from  Molybdenum  and  Tungsten     241 

Separation  from  Palladium,  Platinum,  Selenium  and  Tellurium  ....  242 

Separation  from  Uranium,  Zinc  and  Tin     243 

Silver 244 

Separation  from  Aluminium 244 

Separation  from  Antimony 245 

Separation  from  Arsenic  and  Lead     246 


xii  CONTENTS 

PAGE 

Silver 

Separation  from  Bismuth  and  Platinum    248 

Separation  from  Selenium  and  Zinc 249 

Mercury : 25° 

Separation  from  Aluminium,  Antimony,  Arsenic  and  Tin 250 

Separation  from  Alkaline  Earths,  Magnesium  and  Alkalies  . 

Separation  from  Cadmium,  Cobalt,  Iron,  Manganese  and  Selenium  251 

Separation  from  Tellurium,  Zinc  and  Bismuth 252 

Gold    ...  

Separation  from  Platinum    252 

Separation  from  Palladium    253 

•mum      

Separation  from  Iridium 253 

Antimony   253 

Separation  from  Tin   253 

Separation  from  Arsenic     255 

Separation  from  Tin  and  Arsenic    256 

Separation  from  Bismuth     *. .  .  257 

Zinc    258 

Separation  from  Manganese  and  Aluminium    258 

Separation  from  Lead  and  Bismuth     259 

Cadmium 259 

Separation  from  Aluminium,  Antimony  and  Arsenic    259 

Separation  from  Bismuth  and  Cobalt    260 

Separation  from  Iron 261 

Separation  from  Lead,  Manganese,  Mercury  and  Nickel 262 

Separation  from  Silver   263 

Separation  from  Zinc      264 

Iron    265 

Separation  from  Nickel  and  Cobalt 265 

Separation  from  Zinc    266 

Separation  from  Manganese      267 

Simultaneous  Deposition  of  Iron  and  Manganese  Dioxide 268 

Separation  from  Aluminium      269 

Separation  from  Uranium  and  Chromium 270 

Separation  from  Beryllium 272 

Separation  from  Beryllium  and  Aluminum 273 

Separation  from  Aluminium,  Uranium  and  Rare  Earths  273 

Separation  from  Vanadium    274 

Separation  from  Lead     275 

Nickel   275 

Separation  from  Lead  and  Zinc 275 

Rapid  Separation  from  Zinc      278 

Separation  from  Chromium     279 

Separation  from  Aluminium  and  Uranium   .  280 


CONTENTS  xiii 

PAGE 

Cobalt  280 

Separation  from  Zinc  280 

Separation  from  Aluminium,  Chromium/Uranium  and  Nickel 281 

Lead 284 

Separation  from  Other  Metals 284 

Molybdenum    286 

Separation  from  Vanadium 286 

PART  IV.     SPECIAL  ANALYSES 287 

Analysis  of  Commercial  Copper    287 

Determination  of  Copper  in  Materials  Rich  in  Iron 293 

Analysis  of  Brass     296 

Copper  Matte  (Lead  Matte)     297 

Bronzes    298 

Alloys  of  Lead,  Tin,  Antimony  and  Copper 300 

White  Metals 302 

Analysis  of  Commercial  Zinc     303 

Zinc  and  Zinc  Dust,  Blue  Powder,  Flue  Dust  and  Zinc  Ores 305 

Sphalerite    306 

Lead  (Refined  or  Soft  Lead) 307 

Hard  Lead  and  Crude  Lead 310 

Iron  Ores,  Iron  and  Steel     , 310 

Nickel      311 

Determination  of  Nickel  in  Steel 312 

Chrome-nickel  Steel     314 

Tin 314 

Antimony 316 

Copper-manganese 317 

Manganese  Silicide      318 

Determination  of  Mercury  in  Cinnabar    318 

International  Atomic  Weights 320 

Logarithms 322 

Antilogarithms     ...... 324 


QUANTITATIVE  ANALYSIS 
BY  ELECTROLYSIS 


PART   I. 

INTRODUCTION. 

IN  an  ordinary  gravimetric  analysis,  the  substance  to  be  weighed 
is  formed  by  precipitation  from  a  solution  by  means  of  a  chemical 
reagent.  In  an  electro-analysis  the  substance  to  be  weighed  is 
deposited  by  the  passage  of  an  electric  current  through  the  solu- 
tion. In  gravimetric  analysis  there  are  usually  several  different 
compounds  into  which  the  metal  or  the  acid  may  be  converted. 
The  principal  requirements  to  be  satisfied  by  gravimetric  analysis 
are:  (1)  the  precipitate  shall  contain  only  the  metal  or  acid  to  be 
determined  in  the  form  of  a  known  compound,  i.e.,  it  must  be 
chemically  pure;  (2)  it  must  contain  the  whole  of  the  metal  or 
acid  in  question,  or,  in  other  words,  the  precipitation  must  be 
complete,  and  (3)  the  precipitate  must  be  of  such  a  nature  that 
it  can  be  transformed  easily  into  a  substance  of  known  composi- 
tion from  which  the  quantity  of  the  element  in  question  can  be 
computed  and  in  which  it  remains  unchanged  during  the  weigh- 
ing. If,  moreover,  the  precipitate  possesses  (4)  a  high  molecular 
weight,  and  (5)  if  the  precipitate  settles  quickly  so  that  it  can  be 
filtered  promptly,  it  possesses  two  desirable  properties  which  are 
not,  however,  indispensable. 

The  electrolytic  methods  of  chemical  analysis  are  up  to  the 
present  time  restricted  mainly  to  the  determination  and  separation 
of  metals,  and  as  regards  the  deposits  obtained  a  few  character- 
istics may  be  mentioned.  In  most  cases  the  deposits  consist  of 
the  metal  itself  rather  than  one  of  its  compounds.  Only  a  few 
metals,  such  as  lead,  manganese,  molybdenum  and  uranium,  are 
obtained  in  the  form  of  oxides.  As  with  an  ordinary  gravimetric 
analysis,  it  is  necessary  that  the  deposit  shall  (1)  be  chemically 


BY  ELECTROLYSIS 

pure  and  (2)  contain  all  the  element.  As  regards  the  third  requisite 
of  quantitative  analysis,  which  concerns  the  accurate  weighing, 
it  is  almost  always  true  that  the  metallic  or  oxidic  deposits  are 
easily  converted  by  washing  and  drying  into  a  weighable  condition. 
The  choice  of  a  compound  of  high  molecular  weight  is  naturally 
out  of  the  question,  but  as  regards  the  time  factor  it  is  to-day 
possible  to  carry  out  an  electro-analysis  so  quickly  that  it  is 
finished  in  less  than  an  hour,  or  so  slowly  that  the  deposition  can 
be  completed  during  the  night. 

Just  as  in  carrying  out  an  ordinary  gravimetric  analysis  it  is 
necessary,  for  accurate  work,  to  understand  the  exact  behavior  of 
the  reagents  and  to  know  that  they  are  sufficiently  pure  and 
present  in  sufficient  quantity,  so  in  the  case  of  an  electrolytic 
method  it  is  necessary  to  know  exactly  how  the  electric  current 
behaves  toward  the  solutions,  the  effect  of  different  strengths  of 
current,  and  how  it  is  possible  to  obtain  and  maintain  the  pre- 
scribed current  during  every  operation. 

It  is  necessary,  therefore,  to  find  out,  on  the  basis  of  the  theory 
of  electricity,  what  happens  when  an  electric  current  is  passed 
through  any  given  solution. 

If  the  wires  from  the  positive  and  negative  poles  of  a  suitable 
source  of  current  are  each  connected  with  separate  pieces  of  plati- 
num foil  and  the  two  pieces  of  foil  are  suspended  a  slight  distance 
apart  in  a  sugar  solution  or  in  chloroform,  it  will  be  found,  by  plac- 
ing a  galvanoscope  or  ammeter  in  the  circuit  between  one  of  the 
poles  and  the  wire  that  leads  to  the  liquid,  that  only  a  very  weak 
electric  current  is  flowing. 

If,  however,  the  pieces  of  platinum  foil,  called  the  electrodes, 
are  suspended  in  dilute  sulphuric  acid,  in  dilute  caustic  soda,  or 
in  a  solution  of  sodium  chloride,  the  instrument  will  then  show 
the  passage  of  a  stronger  electric  current.  The  solutions  of  these 
substances  conduct  electricity.  On  the  basis  of  their  behavior 
toward  the  current,  all  soluble  substances  (and  with  these  only 
shall  we  concern  ourselves)  can  be  divided  into  those  which  are 
good  conductors  and  those  which  are  not.  Those  substances 
which,  in  aqueous  solution,  conduct  electricity  are  called  electrolytes; 
to  this  class  belong  most  acids,  most  bases,  and  nearly  all  salts, 
whether  organic  or  inorganic  in  nature,  and  it  is  with  these  that 
electro-analysis  is  concerned. 

The  first  question  that  arises  is  this:  What  changes  take  place 


INTRODUCTION  3 

ie  solution  of  an  electrolyte  when  an  electric  current  is  passed 
through  it? 

If  a  sufficiently  strong  current  is  passed  between  platinum  elec- 
trodes through  a  dilute,  aqueous  solution  of  sulphuric  acid,  or 
through  a  solution  of  potassium  hydroxide,  it  will  be  found  that 
oxygen  gas  is  liberated  at  the  positive  pole  (the  anode)  and  hydro- 
gen gas  at  the  negative  pole  (the  cathode) .  In  the  electrolysis  *  of 
a  solution  of  potassium  sulphate,  oxygen  is  likewise  liberated  at 
the  anode  and  hydrogen  at  the  cathode.  Moreover,  in  this  case, 
blue  litmus  paper  will  show  the  presence  of  acid  in  the  vicinity 
of  the  anode  and  red  litmus  paper  will  enable  one  to  detect  alkali 
in  the  vicinity  of  the  cathode,  although  the  original  solution  of 
potassium  sulphate  was  neutral.  Here,  as  in  the  electrolysis  of 
all  other  electrolytes,  a  decomposition  has  taken  place  in  the 
liquid  owing  to  the  action  of  the  electric  current. 

For  a  long  time  it  was  assumed  that  the  action  of  the  electric 
current  was  to  decompose  the  molecules  of  electrolyte.  Thus, 
for  example,  sulphuric  acid,  H2SO4,  was  supposed  to  be  broken 
down  into  the  components  H2  and  SO4,  the  hydrogen  was  liberated 
at  the  cathode,  while  the  acid  radical,  S04,  which  cannot  exist  by 
itself,  reacted  with  water  so  that  sulphuric  acid  was  again  formed 
and  oxygen  evolved  at  the  anode: 

S04  +  H20  =  H2S04  +  0. 

The  oxygen,  according  to  this  view,  is  not  the  product  of  the 
direct  action  of  the  current  upon  the  acid  but  is  formed  by  the 
action  of  the  group  S04,  which  is  incapable  of  existing  in  a  free 
state,  upon  water;  oxygen,  therefore,  represents  a  secondary  product 
of  the  action  of  the  electric  current. 

The  same  view  applied  to  the  decomposition  of  potassium  sul- 
phate leads  to  these  conclusions:  the  primary  products  of  the 
action  of  the  current  upon  this  salt  are  K2  and  S04;  the  potassium, 
owing  to  its  chemical  nature,  reacts  with  water  as  fast  as  it  is 
set  free 

K2  +  2H2O  =  2KOH  +  H2, 

so  that  potassium  hydroxide  and  hydrogen  appear  at  the  cathode; 

*  Electrolysis  signifies,  in  general,  the  decomposition  of  an  electrolyte  by  the 
influence  of  an  electric  current  irrespective  of  whether  the  substance  itself  is  in 
solution  or  in  a  melted  condition.  The  decomposition  of  the  electrolyte  in  solu- 
tion for  the  purpose  of  analysis  is  appropriately  called  electro-analysis. 


4  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

at  the  anode  the  S04  reacts  with  water,  as  explained  above,  and 
forms  H2S04  with  evolution  of  oxygen. 

This  view  is  not  correct,  according  to  the  views  which  pre- 
vail to-day  concerning  the  nature  of  aqueous  solutions. 

According  to  the  theory  of  electrolytic  dissociation,  proposed 
by  Arrhenius  in  1887  and  since  verified  by  careful  study  of  the 
physical  properties  of  solutions,  it  is  assumed  that  components 
of  the  electrolyte,  which  formerly  were  thought  to  be  formed  by 
the  initial  action  of  the  electric  current,  already  exist  as  such 
in  an  aqueous  solution. 

It  is  not  necessary,  here,  to  discuss  the  basis  of  this  theory 
of  electrolytic  dissociation  or  ionization;  it  is  now  generally 
taught  in  the  study  of  inorganic  chemistry.  It  will  be  well, 
however,  to  review  the  theory  as  far  as  it  pertains  to  the  under- 
standing of  the  mechanism  of  the  changes  that  take  place 
during  electro-analysis. 

Faraday,  who  first  designated  the  positive  electrode  as  the 
anode  and  the  negative  electrode  as  the  cathode,  noticed  that  the 
components  of  the  electrolyte  migrated  toward  one  or  the  other 
of  the  electrodes,  and  therefore  called  these  components  ions 
(wanderers).  The  component  which  moves  toward  the  anode 
(+  pole)  is  called  the  anion  and  that  which  moves  toward  the 
cathode  (-  pole)  is  called  the  cation.  The  anion,  since  it  is 
attracted  to  the  positive  pole,  must  be  regarded  as  the  electro- 
negative constituent  of  the  electrolyte,  and  the  cation,  since  it  is 
attracted  to  the  negative  pole,  must  be  regarded  as  the  electro- 
positive constituent.  The  new  thing  in  the  theory  of  Arrhenius 
consists  merely,  as  stated  above,  in  assuming  that  the  ions  already 
exist  in  aqueous  solutions  and  do  not  result  from  the  action  of 
the  electric  current  upon  the  solution.  In  an  aqueous  solution  of 
sodium  chloride,  for  example,  it  is  assumed  that  sodium  and  chlo- 
ride ions  are  present.  To  bring  this  hypothesis  into  harmony  with 
the  well-known  fact  that  free  sodium  cannot  exist  in  contact  with 
water,  and  furthermore  to  explain  the  fact  that  the  ions  are 
attracted  by  the  electrically  charged  electrodes,  it  is  necessary 
to  ascribe  properties  to  the  ions  which  are  not  attributed  to 
elementary  atoms.  It  is  assumed  that  the  ions  are  atoms,  for 
groups  of  atoms,  which  are  charged  with  e^ctricity,  and  in  the 
sodium-chloride  solution  the  sodium  ions  are  charged  with  positive 
electricity  while  the  chloride  ions  bear  an  equal  charge  of  negative 


INTRODUCTION  5 

electricity.  The  ionic  condition  is  expressed  by  writing  small 
-h  signs  above  the  symbols  of  positively  charged  ions  (cations) 
and  small  —  signs  above  negatively  charged  ions  (anions);*  thus 
the  ionic  condition  of  dissolved  sodium  chloride  is  expressed  by 
Na+  and  Cl~.  The  charges  of  opposite  sign  must  be  equal,  for 
the  entire  solution  is  electrically  neutral. 

By  assuming  the  existence  of  ions,  charged  respectively  with 
positive  and  negative  electricity,  it  is  perfectly  clear  why  the  ions 
migrate  when  subjected  to  the  action  of  an  electric  current;  the 
source  of  the  current  charges  the  positive  electrode  (anode)  with 
positive  electricity  and  this  anode  attracts  the  negatively  charged 
ions  (anions)  and  repels  the  positively  charged  cations;  the  latter 
are  attracted  by  the  negatively  charged  cathode,  which  on  its 
part  repels  the  positively  charged  anions  and  sends  them  toward 
the  anode. 

The  passage  of  the  electric  current  through  the  solution  from 
one  electrode  to  the  other  is  a  purely  physical  change,  involving 
merely  the  migration  of  the  ions.  The  passage  of  electricity 
from  the  metallic  electrode  to  the  solution,  however,  always 
accomplishes  an  electrochemical  change.  This  chemical  change 
is  an  oxidation  at  the  anode  and  a  reduction  at  the  cathode;  the 
two  processes  always  take  place  simultaneously. 

The  term  oxidation  originally  implied  increasing  the  oxygen 
content  of  a  substance  and  reduction  implied  the  removal  of 
oxygen.  In  the  typical  oxidation  of  hydrogen  by  means  of 
oxygen  to  form  water,  we  now  regard  the  hydrogen  as  repre- 
senting the  electro-positive  constituent  and  the  oxygen  as  repre- 
senting the  electro-negative  constituent  of  the  water.  In  the 
oxidation  of  hydrogen  by  oxygen,  therefore,  the  former  is  changed 
from  the  electrically-neutral  to  the  electro-positive  condition  and 
the  latter  from  the  neutral  to  the  electro-negative  state.  In  terms 
of  the  electrolytic  theory,  the  term  oxidation  merely  means  an 
increase  in  the  electro-positive  charge  on  an  atom,  or,  what  amounts 
to  the  same  thing,  a  decrease  in  the  electro-negative  charge.  In 
the  same  way  a  reduction  is  merely  a  decrease  in  the  electro- 
positive charge  associated  with  an  atom  or  an  increase  in  the 
electro-negative  charge. 

*  Instead  of  designating  the  charges  on  the  ions  by  small  -f  and  —  signs, 
many  authorities  use  dots  and  dashes.  Thus  the  ions  of  NaCl  are  often  written 
Na*  and  Cl'. 


6  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Every  species  of  ion  present  in  a  solution  takes  part  in  some 
degree  in  the  movement  toward  the  electrodes  during  electrolysis. 
At  each  electrode  usually  a  single  species  of  ions  is  oxidized  or 
reduced  In  a  tenth-normal  solution  of  sodium  chloride  in 
water  the  salt  molecules  are  ionized  to  the  extent  of  nearly 
85  per  cent  but  the  water  molecules  are  ionized  into  hydrogen 
cations  and 'hydroxyl  anions  only  to  about  0.0000002  per  cent  at 
room  temperature.  Pure  water  is  a  poor  conductor,  but  the  salt 
solution  is  a  good  electrolyte.  In  the  electrolysis  of  a  salt  solu- 
tion between  platinum  electrodes,  it  is  chiefly  the  movement  of  the 
sodium  cations  and  chloride  anions  that  interests  us.  At  the  elec- 
trodes, however,  it  is  another  matter.  It  is  a  great  deal  easier  to 
discharge  hydrogen  ions  at  the  cathode  than  to  discharge  sodium 
ions,  even  although  only  a  few  of  the  former  are  present  at  any 
instant.  The  moment  the  hydrogen  ions  originally  present  are  dis- 
charged, however,  more  of  them  are  formed  from  the  ionization  of 
the  water  which  takes  place  at  a  very  rapid  rate.  This  increases 
the  concentration  of  hydroxyl  ions  in  the  vicinity  of  the  cathode 
because  as  each  hydrogen  atom  is  discharged,  an  equivalent  weight 
of  hydroxyl  ions  remains.  These  hydroxyl  ions  are  in  equilibrium 
with  the  sodium  ions  that  have  migrated  to  the  cathode  region. 

It  was  formerly  customary  to  assume,  in  explaining  the  elec- 
trolysis of  a  sodium  chloride  solution,  that  sodium  was  at  first 
set  free  at  the  cathode  and  that  the  free  sodium  reacted  with  the 
water  to  form  sodium  hydroxide.  This  idea,  however,  is  not  sub- 
stantiated by  the  facts. 

The  chemical  reduction  that  takes  place  at  the  cathode  during 
an  electrolysis,  whether  it  involves  the  deposition  of  a  metal, 
the  evolution  of  hydrogen  gas,  the  loss  in  valence  of  some  pos- 
itively-charged element,  or  the  gain  in  valence  of  some  negatively- 
charged  element,  will  always  be  that  reduction  which  it  is  easiest 
to  accomplish  under  the  prevailing  conditions.  Theoretically, 
under  suitable  conditions,  any  oxidation  and  any  reduction  can 
be  accomplished  by  means  of  electrolysis.  The  conductance  of 
the  solution  is  determined  by  the  number  of  ions  present,  the 
charge  each  ion  bears,  and  the  mobility  of  the  ion.  The  chemical 
nature  of  the  ions,  or  at  least  the  readiness  with  which  they 
are  oxidized  and  reduced,  has  nothing  to  do  with  the  conductance 
of  an  electrolyte.  At  the  electrodes,  however,  although  the 
concentration  of  the  ions  does  have  an  important  effect,  the 


INTRODUCTION  7 

chemical  nature  of  the  ions,  especially  as  regards  their  readiness 
to  be  oxidized  or  reduced,  is  of  great  importance.  It  is  easier 
to  oxidize  hydrogen  ions  from  hydrogen  chloride  solution  than 
from  water  because  there  are  so  many  more  hydrogen  ions  present 
from  the  ionization  of  the  hydrogen  chloride  than  from  the 
ionization  of  water.  But  it  is  so  much  easier  to  reduce  hydrogen 
ions  than  sodium  ions  from  a  salt  solution  that  hydrogen  gas  is 
formed  during  electrolysis. 

It  is  true,  to  be  sure,  that  it  is  possible  to  get  sodium  amalgam 
formed  at  the  cathode  if  the  electrolysis  is  carried  out  with  a 
mercury  cathode.  In  this  case  the  sodium  ions  are  reduced, 
but  the  sodium  is  kept  in  metallic  solution  and  is  not  set  free 
in  the  pure  state.  By  changing  the  conditions  very  slightly 
it  is  possible  to  change  the  nature  of  the  chemical  reactions  that 
take  place. 

All  that  has  been  said  thus  far  refers  to  the  qualitative  side  of 
the  action  of  the  current  upon  an  electrolyte.  To  understand  the 
quantitative  relations  it  is  necessary  to  know  something  about  the 
measurable  factors  of  the  current  which  play  a  part  in  electro- 
analysis,  and  to  know  how  the  measurements  are  made.  These 
factors  are  electromotive  force  (potential),  current  strength,  and 
resistance  and  they  stand  toward  one  another  in  the  relation 
expressed  by  Ohm's  law 

electromotive  force 

current  strength  =  -  ^_^ 

resistance 

E 
or  i  =  - 

This  law  holds,  in  the  first  place,  for  the  passage  of  electricity 
through  a  solid  conductor  (conductor  of  the  first  class);  it  holds 
equally  well,  as  we  shall  soon  see,  for  the  passage  of  electricity 
through  the  solution  of  an  electrolyte  (conductor  of  the  second 
class).  The  distinction  between  these  two  classes  of  conductors 
arises  from  the  fact  that  in  metallic  conductors  (carbon  is  classed 
with  these)  there  is  no  permanent  alteration  of  the  substance 
produced  by  the  passage  of  the  current,  whereas  in  the  case  of 
liquid,  non-metallic  conductors,  a  transformation  of  substance 
takes  place,  as  we  have  already  seen;  a  heating  effect  is  noticeable 
when  the  current  passes  through  either  kind  of 'a  conductor. 

The  unit  employed  for  measuring  the  current  strength,  or  in- 
tensity, is  called  the  ampere  and  it  represents  a  current  which  will 


8  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

deposit  1  118  mgms.  of  silver  in  a  second  from  a  solution  con- 
taining about  15  gms.  of  pure  silver  nitrate  in  85  cc.  of  distilled 

water.* 

The  unit  employed  for  measuring  resistance  is  the  resistance 
offered  at  0°  C.  by  a  column  of  mercury  106.3  cm.  long  weighing 
14.4521  gms.  and  being  1  square  millimeter  in  cross  section;  this 
resistance  is  called  an  ohm. 

The  unit  of  electromotive  force,  or  difference  in  potential,  is 
called  the  volt.  It  represents  the  electromotive  force  which  pro- 
duces a  current  of  one  ampere  in  intensity  through  a  conductor 
having  a  resistance  of  one  ohm.  If  any  two  of  the  above  three 
factors  are  expressed  in  numbers,  the  third  can  be  found  by  the 
equation  of  Ohm's  law  which  reads 

E  volt 

i  =  -,    or    ampere  =  ^- 

Of  these  three  factors,  the  current  strength,  or  intensity,  is 
easiest  to  measure.  It  is  only  necessary  to  insert  an  instrument 
called  an  amperemeter,  or  ammeter,  in  the  circuit  and  the  position 
of  the  needle  on  the  scale  shows  the  number  of  amperes.  The 
question  now  arises:  What  is  the  part  played  in  electro-analysis 
by  the  current  strength? 

If  the  deposition  of  an  element,  e.g.,  a  metal  like  silver,  depends 
upon  the  neutralization  of  the  positive  charge  of  the  silver  ions 
by  the  negative  electricity  of  the  cathode,  then  the  deposition 
of  the  metal  must  take  place  more  rapidly,  i.e.,  in  a  unit  of  time 
so  many  more  ions  must  be  transformed  into  atoms,  in  propor- 
tion as  the  quantity  of  negative  electricity  offered  to  the  positively 
charged  silver  ions  at  the  cathode  is  large.  While  a  current 
of  one  ampere  will  cause  the  deposition  of  1.118  mgm.  of  silver 
from  a  silver  salt  in  a  second,  a  current  of  two  amperes  will  cause 
the  deposition  of  2  X  1.118  mgm.  of  silver  in  the  same  time. 
In  general,  it  holds  for  the  deposition  of  all  substances  that 
the  quantities  deposited  at  the  electrodes  in  a  unit  of  time  are  pro- 
portional to  the  current  strengths. 

This  law  may  be  easily  demonstrated  by  electrolyzing  an 
aqueous  solution  of  copper  sulphate  for  ten  minutes  with  a  current 
of  definite  strength  and  weighing  the  deposited  copper,  then 

The  legal  electrical  units  in  the  United  States  are  defined  in  a  Bulletin 
of  the  U.  S.  Coast  and  Geodetic  Survey,  Dec.  27,  1893. 


INTRODUCTION  9 

repeating  the  experiment,  using  the  same  solution  the  same 
length  of  time  and  a  current  twice  as  strong.  The  weight  of 
copper  deposited  in  the  second  case  will  be  twice  that  obtained 
in  the  first  experiment. 

What  relation  holds  with  regard  to  the  quantities  of  different 
substances  deposited  in  equal  lengths  of  time  by  the  same  current? 
Faraday's  law  *  answers  this  question.  The  quantities  of  different 
substances  deposited  in  equal  lengths  of  time  by  the  same  current 
stand  in  the  same  relation  to  one  another  as  do  their  electrochemical 
equivalents. 

This  law  may  be  demonstrated  experimentally  by  taking  a 
series  of  solutions  containing  the  salts  of  different  metals,  each 
connected  in  series,  and  passing  a  current  through  the  series  of 
solutions  for  a  definite  length  of  time,  and  then  weighing  the 
deposited  metals.  It  will  be  found  that  the  weights  obtained  are 
in  proportion  to  the  equivalent  weights  of  the  substances  in  ques- 
tion. Thus  in  aqueous  solutions  of  silver  nitrate,  cupric  chloride, 
and  ferric  chloride  decomposed  by  one  and  the  same  current,  the 
weights  of  metal  deposited  will  be  as 

Ag+  Cu++  Fe+++ 

107.88    :   ™-         :    «**. 


It  should  be  noticed  particularly  that  the  proportionality  does 
not  refer  to  the  atomic  weights  but  to  equivalent  weights,  i.e., 
to  the  atomic  weights  divided  by  the  valence  of  the  element  in 
question.  In  a  similar  experiment  with  silver  nitrate,  cuprous 
chloride  and  ferrous  chloride,  the  quantities  of  metal  deposited 
would  bear  the  relation: 

Ag+  Cu+  Fe++ 

107.88    :    63.57   :    ^^- 

What  holds  true  of  the  metals,  which  are  easier  to  determine 
experimentally,  holds  equally  true  with  respect  to  the  quantities 
of  anions  discharged  by  the  same  current. 

Faraday's  law,  in  the  light  of  the  ionic  theory,  suggests  a  num- 
ber of  new  consequences.  If  a  current  of  a  certain  intensity  will 

*  The  above-mentioned  law  expressing  the  proportionality  between  the 
quantities  of  the  same  substance  and  the  current  strengths  was  also  discov- 
ered by  Faraday. 


10  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

deposit  107.88  mgms.  of  silver  in  the  same  time  that  63.57  mgms. 
of  copper  are  deposited  from  cuprous  chloride,  and  if,  as  has 
already  been  stated  repeatedly,  the  deposition  is  due  to  the  neutral- 
ization of  opposite  charges  of  electricity  on  the  electrode  and  ion, 
then  the  ions  of  107.88  mgms.  of  silver  must  bear  the  same  charge 
of  positive  electricity  as  are  borne  by  63.57  mgms.  of  copper  ions; 
for  the  current  of  the  given  strength  must  carry  to  the  cathode  in 
the  silver  solution,  in  the  same  length  of  time,  the  same  quantity 
of  negative  electricity  that  it  carries  to  the  cathode  in  the  copper 
solution.  The  same  quantities  of  electricity  are  carried,  therefore, 

by  63'5I  mgms.  of  cupric  ions,  by  — ^—  mgms.  of  ferric  ions,  and 

K.K.  QPi 

by  °°       mgms.  of  ferrous  ions. 

In  general  it  may  be  said  that  equivalent  quantities  of  univalent 
ions  (e.g.,  107.88  Ag  and  63.57  Cu)  carry  the  same  quantities  of 
electricity,  or,  in  other  words,  all  univalent  ions  bear  equal  electric 
charges.  As  regards  the  charges  on  polyvalent  ions,  let  us  imagine 
that  a  current  of  one  ampere  is  passing  through  a  solution  con- 
taining cuprous  ions  and  through  another  containing  cupric  ions. 
At  the  end  of  a  certain  period,  this  current  will  deposit  63.57  mgms. 

,  63.57  . 

of  copper  from  the  cuprous  solution  and  — = —  mgms.  of  copper 

from  the  cupric  solution.  Inasmuch  as  cupric  ions  have  the  same 
weight  as  cuprous  ions,  it  follows  that  the  number  of  cupric  ions 

/*O     Fv7 

which  by  their  discharge  have  yielded  — ^—  mgms.  of  copper  must 

2 

be  half  as  large  as  the  number  of  cuprous  ions  from  which  63.57 
mgms.  of  copper  have  been  deposited.  The  current  strength  was 
the  same  in  both  solutions,  i.e.,  the  quantities  of  electricity  re- 
quired to  deposit  these  two  different  quantities  of  copper  were 
the  same.  It  follows  from  this,  and  from  what  was  said  above, 
that  one  half  as  many  cupric  ions  bear  the  same  electric  charge  as 
a  given  number  of  cuprous  ions,  or,  that  the  charge  borne  by  a 
cupric  ion  is  twice  that  of  a  cuprous  ion.  This  is  designated  hi  the 
symbol  by  two  +  signs:  Cu++  Trivalent  ions  bear  a  triple 
charge,  e.g.,  Fe+4  +  is  the  symbol  of  the  ferric  ion.  The  same  holds 
for  the  charges  on  the  anions;  the  charge  is  shown  by  the  number 

-  signs:  S04  ~  is  the  symbol  of  the  sulphate  ion  and  P04" 
of  the  phosphate  ion. 


INTRODUCTION  11 

On  page  7,  it  was  stated  that  a  current  of  one  ampere  would 
deposit  1.118  mgms.  of  silver  from  the  aqueous  solution  of  a  silver 
salt.  Using  this  number  as  a  basis,  it  is  possible  to  compute  the 
strength  of  the  current  from  the  weight  of  silver  deposited  in  a 
definite  time. 

Besides  the  conception  of  current  strength,  which  is  deter- 
mined by  the  quantity  of  electricity  flowing  during  a  unit  of 
time  (one  second)  there  is  another  unit  for  measuring  the  quantity 
of  electricity  called  the  coulomb.  A  coulomb  is  the  quantity 
of  electricity  transferred  by  a  current  of  1  ampere  in  one  second. 
If  a  current  of  a  amperes  flows  for  a  period  of  t  seconds,  then 
the  quantity  of  electricity  that  passes  through  the  circuit  during 
that  period  is  a  X  t  coulombs. 

From  these  definitions  it  is  easy  to  compute  how  many  coulombs 
of  electricity  are  necessary  to  discharge  a  gram-equivalent  of 
a  metal.  Silver,  a  univalent  metal,  has  an  atomic  weight  of 
107.88,  this  is  the  value  of  the  gram  equivalent  of  silver  because 
the  metal  is  univalent  in  its  salts.  Since,  by  definition,  1  coulomb 
of  electricity  deposits  0.001118  gm.  of  this  metal,  it  will  take 

1 07  88 
0  001118  =  96,500  coulombs  to  discharge  a  gram  equivalent  of 

silver. 

This  quantity  of  electricity,  moreover,  corresponds  not  only 
to  the  charge  on  a  gram  equivalent  of  silver,  but  it  corresponds 
to  the  unit  charge  on  a  gram  atom  of  any  other  element.  96,500 
coulombs  will  discharge  63.57  gms.  of  copper  from  a  solution  of 

a  cuprous  salt,  but  only  — -^ —  gms.  of  copper  from  a  cupric  salt. 

To  reduce  an  atomic  weight  of  iron  from  the  ferric  to  the  ferrous 
state  requires  at  the  cathode  96,500  coulombs  of  electricity; 
to  reduce  the  same  weight  of  ferric  iron  to  metal  requires  3  X 
96,500  coulombs  of  negative  electricity.  The  electrochemical 
equivalent  of  iron,  therefore,  is  the  atomic  weight  if  the  con- 
ditions are  such  at  the  cathode  that  the  ferric  salt  is  merely 
reduced  to  ferrous  salt  but  the  electrochemical  equivalent  is  only 
one-third  the  atomic  weight  if  the  iron  is  deposited  as  such  upon 
the  cathode. 

The  number  96,500  represents,  therefore,  the  electrochemical 
unit  for  the  quantity  of  electricity;  it  is  called  the  Faraday 
and  is  denoted  by  the  symbol  F  (1  F  =  96,500  coulombs  =  26.82" 


12  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

ampere-hours).  It  has  already  been  stated  that  an  oxidation 
always  takes  place  at  the  anode  simultaneous  with  the  reduction 
at  the  cathode  and  it  is  easy  to  compute  how  much  electricity 
is  necessary  to  accomplish  any  given  oxidation.  The  liberation 
of  oxygen  at  the  anode  during  electrolysis  corresponds  to  the 
neutralization  of  a  double  negative  charge  on  the  oxygen  atom. 
If  this  is  the  only  oxidation  taking  place  at  the  anode,  one 

ri  4-      Tiff" 

Faraday  passing  through  the  circuit  will  liberate      '2       =  8  gms. 

of  oxygen  gas.  The  oxidation  of  the  chromium  in  a  chromic 
salt  to  a  chromate  at  the  anode  corresponds  to  increasing  the 
positive  charge  on  the  chromium  atom  from  3  to  6  and  the 

electrochemical  equivalent  of  chromium  is  therefore  —  '-^— 
17.33  gms. 

To  find  how  many  coulombs  of  electricity  are  required  to 
accomplish  any  desired  reduction  at  the  cathode  or  oxidation  at 
the  anode,  first  determine  what  change  in  valence  takes  place 
and  remember  that  to  impart  a  unit  charge  of  electricity,  or  to 
neutralize  a  unit  charge,  96,500  coulombs  are  required.  To 
determine  how  long  it  will  take,  divide  the  coulombs  required 
by  the  amperage  of  the  current  used.  For  example,  how  long 
will  it  take  a  current  of  4  amperes  to  deposit  3  gms.  of  nickel 
from  a  solution,  assuming  this  to  be  the  only  reaction  that  takes 
place  at  the  cathode?  The  atomic  weight  of  nickel  is  58.68  and 
its  valence  is  2.  The  computation  is  as  follows: 

3  X  2  X  96,500 

=  4,934  seconds. 


Similarly,  with  the  help  of  value  F,  the  weights  of  different 
metals  that  will  be  deposited  per  second  by  a  current  of  1  ampere 

can  be  computed.    Thus  1  coulomb  will  deposit 


, 

587  gms.  of  zinc  because  zinc  is  bivalent  and  the  Faraday 
corresponds  to  the  charge  residing  on  half  the  atomic  weight 
in  grams  of  this  metal.  For  any  given  time,  t  seconds,  and  any 
given  current,  a  amperes,  it  is  only  necessary  to  multiply  the  above 
number  by  t  and  n. 

For  the  deposition  of  iron  from  a  solution  of  a  ferrous  salt, 
a  corresponding   computation   gives   the   value   0.0002894   gm. 


MIGRATION  OF  THE  IONS  13 

as  the  quantity  of  metal  deposited  by  1  coulomb  of  electricity. 
From  the  solution  of  a  ferric  salt,  the  iron  value  is  0.0001929. 

It  must  be  mentioned  here,  however,  that  the  computation, 
with  the  help  of  Faraday's  law,  of  the  quantities  of  metal  de- 
posited will  give  the  values  actually  obtained  in  an  experiment 
only  when  all  the  current  flowing  through  the  solution  is  used  for 
the  discharge  of  the  ions  of  the  metal  in  question.  This  is  not 
usually  the  case  in  electro-analysis,  as  will  be  shown  later.  In 
most  cases  some  hydrogen  ions  are  discharged  while  the  metal  is 
being  deposited  and  in  this  way  a  part  of  the  current  is  not  utilized 
for  precipitating  the  metal.  The  current  yield,  which  is  based 
upon  the  quantity  of  current  actually  used  for  depositing  the 
metal  itself,  is  in  such  cases  smaller  than  the  theoretical  value 
computed  with  the  aid  of  Faraday's  law;  on  the  other  hand,  the 
sum  of  the  weights  of  all  the  ions  discharged  exactly  corresponds 
to  the  law. 

Two  other  units  are  of  interest  in  connection  with  electrical 
measurements,  the  unit  of  work  and  the  unit  of  power.  The 
unit  of  work  is  the  joule.  It  is  equivalent  to  107  ergs  and  is 
practically  equivalent  to  the  energy  expended  in  one  second  by 
an  ampere  against  the  resistance  of  an  ohm.  If  the  quantity 
of  electricity  is  expressed  in  coulombs  and  the  electromotive  force 
in  volts,  the  product  will  be  volt-coulombs  (volts  X  amperes  X 
time  in  seconds)  or  joules.  In  commercial  work,  the  unit  of 
power  is  the  watt  (or  the  kilowatt,  which  is  1000  times  as  large) 
which  represents  work  done  at  the  rate  of  one  joule  per  second. 
Multiplying  the  voltage  by  the  amperage  gives  watts  and  multiplying 
this  by  the  time  in  seconds  gives  watt-seconds  or  joules. 

Migration  of  the  Ions. 

If  an  electric  current  is  conducted  through  a  solution  of  cuprous 
chloride,  CuCl,  it  is  evident,  from  what  has  been  said,  that  for 
each  63.57  gms.  of  copper  deposited  upon  the  cathode  35.46  gms. 
of  chlorine  ions  will  be  discharged  at  the  anode.  As  soon  as  some 
of  the  copper,  or  chlorine,  is  transformed  into  the  electrically 
neutral  condition  at  the  electrode,  new  ions  of  the  same  kind  must 
appear  at  each  electrode  as  otherwise  all  the  copper,  or  chlorine, 
will  never  be  removed  from  the  solution.  The  ions  which  are 
originally  distributed  uniformly  throughout  the  entire  solution 


14  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

must  migrate,  even  from  the  most  distant  parts  of  the  solu- 
tion, toward  the  electrodes,  the  positively  charged  ions  mov- 
ing toward  the  cathode  and  the  negatively  charged  ions  moving 
in  the  opposite  direction  toward  the  anode.  The  discharge  at 
the  two  electrodes  must  end  at  the  same  time,  for  when  all  the 
copper  ions  have  been  discharged  there  can  remain  no  negatively 
charged  chlorine  ions  because  the  solution  itself  at  no  time  pos- 
sesses electrical  properties.  The  simplest  assumption,  therefore, 
would  be  that  the  cuprous  ions  and  chlorine  ions  migrate  with 
equal  velocities  toward  the  opposite  electrodes.  Experiment 
shows,  however,  that  this  is  not  true. 

Let  us  imagine,  using  the  illustration  suggested  by  LeBlanc, 
that  the  solution  of  an  electrolyte,  such  as  hydrochloric  acid,  is 
divided  into  three  compart- 
ments (Fig.  1),  of  which  the 
walls  C  and  D  are  easily 
penetrable  by  the  ions.  The 
solution  contains  30  gram 
equivalents  of  HC1,  and,  as- 
suming a  homogeneous  mix- 
ture, each  compartment  contains  10  gram  equivalents  of  HC1.  The 
wall  A  forms  the  anode  and  the  wall  K  the  cathode.  If  a  current 
is  passed  through  the  solution,  we  know  that  1  F,  or  96,500  cou- 
lombs, will  decompose  1  gram  equivalent  HC1  (cf.  p.  11)  discharg- 
ing at  K,  1  gram  equivalent  H+  and  at  A,  1  gram  equivalent  Cl~, 
so  that  after  the  passage  of  this  quantity  of  electricity,  the  entire 
solution  will  contain  29  gram  equivalents  of  HCL 

The  middle  compartment  CD  merely  serves  as  a  passageway 
between  the  two  electrodes  and  no  change  takes  place  within  it. 
At  K,  1  gram  equivalent  of  H+  has  left  the  solution;  at  A,  1  gram 
equivalent  of  Cl~.  If,  now,  there  were  no  other  changes  in  the 
compartments  AD  and  KC,  there  would  be  present,  besides  the 
unchanged  9  gram  equivalents  of  HC1, 1  gram  equivalent  of  H+  in 
AD,  and  1  gram  equivalent  of  Cl~  in  KC.  At  the  electrodes, 
however,  it  is  not  possible  for  free  ions  to  be  present,  as  in  that 
case  the  solution  in  the  compartments  AD  and  KC  would  possess 
free  electricity,  whereas  in  reality  it  is  neutral.  This  electrically 
neutral  condition  can  be  brought  about  only  by  the  movement 
of  some  of  the  H+  in  AD  toward  K  and  of  some  of  the  Cl~  in 
KC  toward  A. 


MIGRATION  OF  THE  IONS  15 

If  we  assume,  as  actually  happens,  that  the  H+  ions  migrate 
five  times  as  fast  as  the  Cl~  ions,  or,  in  other  words,  that  5  H+ 
enter  the  compartment  KC  while  1  Cl~  enters  the  compartment 
AD,  then  of  the  original  1  gram  equivalent  of  H+  in  AD,  %  gram 
equivalent  will  have  migrated  toward  K  and  |  gram  equivalent 
will  have  remained  behind.  Meanwhile,  of  the  residual  Cl~  in 
KC,  J  gram  equivalent  has  migrated  toward  A  and  is  in  equilib- 
rium in  the  compartment  AD  with  the  J  gram  equivalent  H+ 
that  remained  there,  and  forms  £  gram  equivalent  HC1.  In  this 
way  the  electrically  neutral  condition  of  the  solution  in  AD  is 
explained;  there  are  now  present  in  AD  9£  gram  equivalents  of 
HC1. 

After  the  migration  of  the  J  gram  equivalent  of  Cl~  from  KC 
toward  A,  there  still  remains  £  gram  equivalent  of  Cl~  in  KC, 
but  these  ions  are  in  equilibrium  with  the  f  gram  equivalent  of 
H+  that  has  migrated  from  K  so  that  an  electrically  neutral  con- 
dition likewise  prevails  in  KC',  the  solution  there  now  contains 
9f  gram  equivalents  of  HC1. 

Although  no  change  in  concentration  has  taken  place  in  the 
middle  compartment,  such  changes  have  occurred  in  the  end  com- 
partments: AD  now  contains  9J  gram  equivalents  of  HC1,  KC 
contains  9f  gram  equivalents  of  HC1  and  CD  still  contains,  as  at 
first,  10  gram  equivalents  of  HC1. 

Conversely,  from  the  observed  fact  that  the  concentration  of 
HC1  in  one  of  the  end  compartments  is  different  from  that  of  the 
other,  the  conclusion  can  be  drawn  that  the  ions  migrate  with 
different  velocities  and  from  the  changes  in  concentration  the 
ratio  of  the  velocities  of  migration  can  be  computed. 

This  example  merely  serves  to  impart  some  idea  of  what  is 
meant  by  the  different  migration  velocities  of  the  ions.  The 
description  of  the  methods  and  apparatus  used  to  carry  out  such 
measurements  is  outside  the  scope  of  this  book. 

In  spite  of  the  different  velocities  with  which  the  ions  migrate 
within  the  electrolyte,  the  quantities  of  the  substances  discharged 
at  the  two  electrodes  are  always  equivalent.  This  is  due  to  the 
fact  that  the  quantity  of  positive  electricity  which  reaches  the 
electrodes  in  a  unit  of  time  from  the  source  of  the  current, 
and  which  we  may  designate  as  n  coulombs,  is  "at  once  neu*- 
tralized  at  the  anode  by  n  coulombs  of  r  egative  electricity  on 
the  anions,  and,  likewise,  at  the  cathode  n  coulombs  of  nega- 


16  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

electricity  are  neutralized  by  n  coulombs  01  positive  elec- 
tricity on  the  cations.  The  quantities  of  substance  discharged 
at  the  electrodes,  therefore,  are  independent  of  the  rates  at 
which  the  ions  move  through  the  solution  and  are,  according 
to  Faraday's  law  (p.  9),  directly  proportional  to  the  current 

strength. 

The  cause  of  the  different  velocities  of  migration  lies  in  the 
different  degrees  of  friction  which  the  ions  have  to  overcome  in 
their  passage  through  the  electrolyte.  This  resistance  must  vary 
with  the  different  ions,  according  to  their  nature,  and  when  we 
take  into  consideration  the  extremely  small  masses  that  the  ions 
possess  we  can  see  that  it  must  be  considerable.  For  sake  of 
comparison  one  needs  only  to  recall  how  slowly  a  finely  powdered 
substance,  or  precipitate,  settles  in  a  liquid. 

The  electrical  resistance  of  the  electrolyte  is  not  to  be  confused 

with  this  friction  which  the  ions  have  to  overcome. 

« 

Resistance. 

If  a  copper  wire  is  placed  in  circuit  with  an  ammeter  between 
the  poles  of  a  constant  source  of  current,  the  instrument  will  show 
a  certain  current  strength  in  amperes.  If  the  copper  wire  is 
replaced  by  an  iron  wire  of  the  same  length  and  diameter,  the 
instrument  will  then  show  a  weaker  current.  Conductors  made 
of  different  metals,  but  of  the  same  dimensions,  offer  different 
resistances  to  the  current,  or,  as  is  usually  stated,  different  metals 
have  a  different  conductance  toward  electricity.  Resistance  and 
conductance  are  reciprocal  quantities.  The  different  conduct- 
ance of  metallic  conductors  has  a  bearing  upon  electrolytic  prac- 
tice, for,  in  arranging  the  electrolytic  circuit,  good  conductors 
should  be  chosen,  since  an  increased  resistance  causes,  in  accord- 
ance with  Ohm's  law  (p.  7),  a  weakening  of  the  current  and 
consequently  a  loss  in  energy. 

The  electrolytes  themselves  show  similar  differences  with 
respect  to  conductance.  If  the  current  is  allowed  to  pass  through 
a  concentrated,  neutral  solution  of  copper  sulphate  and  again 
through  the  same  solution  after  it  has  been  acidified  with  sul- 
phuric acid,  the  ammeter  will  show  a  stronger  current  in  the  latter 
case  than  in  the  former.  Since,  in  utilizing  the  current  in 
electrolytes,  a  weakening  of  the  current  results  in  a  loss  of 


RESISTANCE  17 

energy,  it  is  evident  that  the  resistance  of  electrolytes  must 
play  an  important  part  in  electrolysis.  In  the  metallic  part 
of  the  circuit  (the  wires  that  carry  the  current)  the  intensity 
of  the  current  can  be  increased,  as  the  formula  for  Ohm's  law 
shows  (p.  7), 

'-* 

by  making  B  smaller  (e.g.,  using  shorter  wires,  larger  wires,  or 
wires  of  a  metal  that  conducts  better)  or  by  increasing  the  electro- 
motive force  E.  These  two  expedients  are,  to  be  sure,  at  one's 
disposal  in  electro-analysis;  but  in  practice  one  is  confined  within 
narrow  limits. 

A  lessening  of  the  resistance  by  diminishing  the  length  and  cross 
section  of  the  electrolyte  would  result  from  bringing  the  electrodes 
nearer  together  and  exposing  a  larger  electrode  surface.  In 
accomplishing  such  changes,  the  shape  of  the  apparatus  also 
comes  into  consideration. 

Increasing  the  electromotive  force  is  out  of  the  question  in 
many  cases,  because,  as  we  shall  find  later,  it  is  often  necessary  to 
carry  out  the  electro-analysis  under  a  constant  potential.  Even 
when  it  is  necessary  to  maintain  a  certain  potential,  however, 
there  remains  the  possibility  of  increasing  the  intensity  of  the 
current,  and  thereby  accelerating  the  operation,  by  adding  certain 
substances,  which,  as  in  the  above  example,  serve  to  increase  the 
conductivity  of  the  solution.  The  nature  of  the  substance  added 
depends  upon  the  chemical  nature  of  the  electrolyte  and  must 
be  determined  by  experiment.  Sometimes  acids,  sometimes 
alkalies  and  often  salts  may  be  added.  A  fundamental  require- 
ment, which  is  independent  of  the  nature  of  the  metal  to  be  de- 
posited, may  be  stated  as  follows,  —  a  substance  added  to  assist 
in  the  electrolysis  of  a  solution  must  be,  when  dissolved,  a  good 
conductor  of  the  current  and  must  form  no  decomposition  products 
which  are  insoluble  or  in  any  way  detrimental  to  the  analysis. 
Alkalies  and  acids,  which  after  their  decomposition  are  regenerated 
at  the  electrodes,  as  well  as  organic  acids  which  form  gaseous 
decomposition  products,  are  frequently  suitable.  This  last  con- 
dition, together  with  the  marked  solvent  effect  that  oxalic  acid 
exerts,  owing  to  the  formation  of  double  salts,  has  caused  this 
acid  to  find  widespread  application  in  electro-analysis. 


18  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

In  carrying  out  an  electrolysis,  it  is  not  usually  necessary  to 
know  the  resistance  of  the  bath;  in  certain  cases,  however,  it  is 
very  desirable  to  know  the  resistance  or 
conductance  of  an  electrolyte.  For  this 
reason,  the  usual  method  for  measuring 
the  resistance  of  a  liquid  will  be  outlined. 
It  will  be  assumed  that  the  reader  already 
understands  how  the  resistance  of  a  me- 
tallic conductor,  e.g.,  a  wire,  is  measured 
with  the  aid  of  the  Wheatstone  bridge. 
It  will  be  recalled,  on  inspecting  the  dia- 
gram shown  in  Fig.  2,  that  in  the  system 
of  resistances  x,  R,  a  and  6,  one  of  the 
resistances,  R,  can  be  regulated  so  that 
the  current  from  the  source  s  has  no 
effect  upon  the  galvanoscope  G  while 
flowing  through  the  system.  From  the  three  known  resistances 
R,  a  and  b  the  unknown  resistance  x  can  be  computed  from  the 

proportion 

x  :  R  =  a  :  o, 

from  which  it  follows  that 


FIG.  2. 


This  method  cannot  be  applied  directly  to  the  measurement  of 
the  resistance  of  an  electrolyte,  by  simply  inserting  the  liquid, 
with  its  two  platinum  electrodes,  in  place  of  the  resistance  x,  for 
the  case  here  is  somewhat  different.  The  current  in  passing 
through  the  electrolyte  not  only  has  to  overcome  the  resistance 
of  the  liquid  (Ohm's  resistance)  but  it  also  has  to  perform  chemical 
work,  or,  expressed  more  exactly,  to  transport  material.  This 
chemical  work  can  be  avoided  by  using  an  alternating  current 
instead  of  a  direct  current.  Then  the  anode  and  cathode  will 
exchange  places  with  each  reversal  of  the  current;  the  changes 
produced  at  the  electrodes  will  thus  be  reversed  at  each  reversal 
of  the  current,  and,  as  the  latter  takes  place  very  frequently  during 
each  second,  it  is  fair  to  assume  that  practically  no  work  is  ac- 
complished in  the  electrolyte.  Such  an  alternating  current  has 
just  as  little  effect  upon  the  magnetic  needle  of  a  galvanoscope  as 
it  has  upon  the  composition  of  the  solution.  It  is  necessary, 
therefore,  to  use,  instead  of  the  galvanoscope,  an  instrument  which, 


RESISTANCE   , 


19 


as  the  resistance  is  varied,  will  show  the  diminution  and  finally  the 
cessation  of  the  alternating  current;  such  an  instrument  is  the 
telephone. 

In  the  above  diagram,  which  illustrates  the 
use  of  the  Wheatstone  bridge,  the  source  of 
the  current  is  replaced  by  a  small  induction 
coil  (Fig.  3)  the  secondary  current  of  which 
(an  alternating  current)  is  sent  through  the 
four  resistances.  T  is  a  telephone.  In  place 
of  the  resistance  x,  the  solution  of  the  electro- 
lyte is  inserted  with  its  two  platinum  electrodes, 
and  in  place  of  the  resistances  a  and.  6,  a 
platinum  wire  is  used  bearing  a  sliding  con- 
r  5^,  tact  (the  arrow  in  the  diagram).  Instead  of 

)  changing  the  resistance  R,  the  resistances  a  and 

6  are  changed  by  moving  the  point  of  contact 
along  the  wire.     This  contact  is  moved  back 
and  forth  until  the  position  which  produces  a  minimum  tone  in 
the  telephone  is  found.      There  then  exists,  between  the  four 
resistances,  the  equation 


Fig.  4  illustrates  the  complete  instrument  as  designed  by  Kohl- 
rausch. 


W 


FIG.  4. 


It  is  often  desired  to  know  the  resistance  of  a  given  metallic  con- 
ductor, e.g.,  a  wire;  it  is  then  merely  necessary  to  take  the  wire,  or  a 
known  length  of  it,  and  measure  the  resistance  with  the  Wheatstone 


20  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

bridge.  If,  however,  it  is  desired  to  learn  the  specific  resistance  of  the 
metal,  i.e.,  the  resistance  of  a  cube  having  1  cm.  edges,  this  value 
must  be  computed  from  the  experimental  results  by  taking  into  con- 
sideration the  fact  that  the  resistance  is  proportional  to  the  length 
and  inversely  proportional  to  the  cross  section  of  the  conductor. 

The  resistance  of  an  electrolyte,  or  its  reciprocal  value  known  as 
the  conductance,  is  something  which  of  itself  is  seldom  of  interest. 
The  knowledge  of  the  specific  conductance  is  much  more  important. 

If  we  imagine  a  liquid  in  the  form  of  a  cube  each  edge  of  which 
measures  1  cm.  and  two  opposite  faces  of  which  form  the  elec- 
trodes, then  its  resistance,  expressed  in  ohms,  is  termed  the  specific 
resistance  of  the  liquid.  Calling  this  resistance  R«  expressed  in  ohms. 
then  the  specific  conductance  is 

L'  =  F. 

expressed  hi  reciprocal  ohms.  If  the  latter  unit  is  constructed  on 
the  same  basis  as  that  of  the  ohm  (p.  7),  then  it  represents  the  con- 
ductance of  a  liquid  contained  in  a  cube,  with  1  cm.  edges,  having 
the  resistance  of  1  ohm.  Then 


B, 

A  fifth-normal  sulphuric-acid  solution  has  a  specific  conductance 
of  approximately  1  at  40°  (cf.  p.  29). 

The  conductance  of  most  electrolytes  increases  as  the  tem- 
perature is  raised;  with  metallic  conductors  the  conductance 
diminishes  with  rise  of  temperature.  As 
regards  the  dependence  of  conductance 
upon  the  dimensions  of  the  conductor,  it  is 
true  of  electrolytes,  as  of  metals,  that  the 
conductance  diminishes  with  increasing 
depth  (length,  with  metallic  conductors) 
and  increases  as  the  cross  section  or  dis- 
tance between  the  electrodes  increases. 
The  conductance  of  electrolytes,  however, 

also  depends  upon  the  concentration.     As     F~AlJ—  it I— Jo' 

was   mentioned   on  page    17,   one   is  re- 
stricted, in  working  with  liquids,  to  the 
dimensions  of  the   apparatus,  and,  since 
the  concentrations  of  the  electrolytes  may   be  very   different, 
;  is  desirable  to  introduce,  for  purposes  of  comparison,  a  new 


FlG>  5' 


ELECTROMOTIVE  FORCE  OR  POTENTIAL       21 

conception,  namely,  that  of  equivalent  conductance.  Let  us 
imagine  a  rectangular  vessel  (Fig.  5)  constructed  so  that  the  two 
opposite  faces  ABCD  and  A'B'C'D'  lie  1  cm.  apart  and  these 
two  side  faces  serve  as  electrodes,  being  made,  for  example,  of 
platinum.  The  vessel  contains  v  cc.  of  a  solution  in  which  1  gram 
equivalent  of  a  substance  is  dissolved.  The  resistance  of  1  cc.  of 
this  solution  is  its  specific  resistance  R,,  and  its  specific  conduc- 
tivity is  L,  =  —  (cf.  p.  20).  If,  now,  we  imagine  an  electric  cur- 

R« 

rent  passing  through  the  entire  solution,  in  such  a  way  that 
it  enters  through  the  face  ABCD  and  leaves  through  the  face 
A'B'C'D',  then  the  resistance  offered  by  the  whole  solution  is 
v  times  smaller  because  the  cross  section  of  ABCD  is  v  sq.  cm.; 
consequently,  the  conductivity  is  v  times  as  great  as  that  of  1  cc. 
of  the  solution.  This  conductance  is  known  as  the  equivalent  con- 
ductance A,  i.e.,  it  is  the  conductance  between  electrodes  1  cm. 
apart  of  that  volume,  v,  of  the  solution  which  contains  1  gram 
equivalent  of  the  substance.  Expressed  in  an  equation, 

A  =  VLS. 

The  specific  conductance  La  (the  reciprocal  of  the  specific 
resistance  R.)  is  determined  experimentally,  and  to  compute  the 
value  of  A,  the  equivalent  conductance  of  the  solution,  the  value 
L,  is  multiplied  by  the  number  of  cubic  centimeters  in  which  1 
gram  equivalent  of  the  substance  would  be  contained  at  the  given 
concentration.  Thus  a  solution  containing  100  gms.  HC1  in  2 
liters  would  contain  1  gram  equivalent  HC1  (or  36.46  gms.  HC1) 
in  729.2  cc.,  for 

gm.  HC1        cc. 

100  :  2000  =  36.46  :  x, 
x  =  729.2  cc., 

and  the  equivalent  conductance  of  such  a  solution  would  be 

A  =  729.2  La, 

Electromotive  Force  or  Potential.* 

Electro-analysis  had  met  with  remarkable  success  before  the 

significance  of  the  electromotive  force  or  potential  was  recognized. 

I  After  the  first  purely  empirical  methods,  with  galvanic  cells  and 

*  Potential  in  electricity  is  analogous  to  temperature,  and  as  heat  tends  to 
-.  pass  from  a  point  at  a  higher  to  one  at  a  lower  temperature,  so  electricity  tends 
i  to  move  from  a  higher  to  a  lower  potential.  The  electromotive  force  is  a  result 


22  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

no  measuring  instruments,  had  been  abandoned,  the  chief  stress 
was  laid  upon  the  current  strength  and  especially  upon  the  current 
density  at  the  cathode,  i.e.,  the  current  strength  per  100  sq.  cm. 
of  cathode  surface.  The  most  favorable  conditions  for  the  dep- 
osition of  a  metal  were  determined  experimentally  and  the 
directions  for  carrying  out  the  analysis  were  given  with  a 
statement  of  the  current  density  as  the  most  important  of  the 
observed  conditions.  To  be  sure  the  potential  of  the  bath  as  well 
was  usually  given.  The  directions  applied  to  special  apparatus 
(dishes,  cones,  etc.)  and  to  a  particular  composition  of  the  elec- 
trolyte. The  current  strength  was  adjusted  by  inserting  an 
adjustable  resistance  between  the  source  of  the  current  and  the 
electrolytic  bath.  When  an  electrolysis  is  started,  under  these 
conditions,  with  a  definite  current  strength,  it  will  be  found  in 
most  cases  that  the  current  diminishes  gradually  in  strength  as 
the  metal  is  being  deposited.  If,  now,  it  is  desired  to  increase  the 
current,  toward  the  end  of  the  operation,  so  that  the  rate  of 
deposition  will  be  hastened,  then  with  such  an  arrangement  it 
only  possible  to  do  this  by  increasing  the  electromotive  force 
well,  for  the  current  strength  and  electromotive  force  are  mutually 
dependent  upon  one  another.  Often  excellent  results  are  obtaim 
but  this  is  due  to  the  fact  that  good  deposits  of  metal,  and  sorm 
times  good  separations  even,  can  be  obtained  when  the  two  factors, 
current  strength  and  electromotive  force,  vary  throughout  a  con- 
siderable range.  In  such  cases,  the  deposition  of  the  last  tn 
of  metal  can  be  accelerated  by  increasing  the  current  strength 
without  injuring  the  quality  or  purity  of  the  deposit.  In  other 
cases,  disturbances  are  likely  to  result;  either,  in  a  simple  deter- 
mination, the  deposit  becomes  spongy,  or,  in  a  separation,  it 
becomes  contaminated  with  the  metal  from  which  the  separation 
is  to  be  made.  Strictly  speaking,  every  electrodeposition  of  a 
metal  includes  a  separation,  for  under  certain  conditions  hydro- 
gen is  likely  to  be  set  free  at  the  cathode  together  with  the  metal 
to  be  determined.  In  fact  the  electric  discharge  of  hydrogen  ions 
together  with  the  ions  of  the  metal  is  the  cause  of  many  bad  de- 
posits, for  the  gas  tends  to  form  a  hydride  with  the  metal,  and  the 
subsequent  breaking  up  of  the  hydride  loosens  up  the  surface 

of  a  difference  in  potential,  but  as  both  electromotive  force  and  potential  are 
measured  in  volts,  and  both  have  the  same  numerical  value,  the  three  terms 
sciential  drop,  electromotive  force,  and  voltage  are  used  synonymously. 


ELECTROMOTIVE  FORCE  OR  POTENTIAL       23 

the  deposit,  making  it  spongy.  In  some  cases  the  simultaneous 
discharge  of  hydrogen  ions  does  no  harm.  The  liberation  of 
hydrogen  while  the  metal  is  being  deposited  is  accomplished  in 
the  same  way  that  any  two  metals  may  be  deposited  simultane- 
ously; in  both  cases  the  electromotive  force  is  too  great  to  permit 
a  separation. 

Kiliani  (1883)  was  the  first  to  point  out  the  significance  of  the 
electromotive  force  in  electrolysis. 

It  will  be  necessary,  in  order  to  explain  the  nature  and  sig- 
nificance of  the  electromotive  force,  to  go  into  this  matter  a  little 
more  deeply  and  to  consider  how,  according  to  the  prevailing 
theory,  this  force  originates;  for,  as  we  shall  subsequently  find, 
not  only  does  the  electromotive  force,  or  potential,  play  an  im- 
portant part  in  electro-analysis  but  there  results  a  second  electro- 
motive force,  called  polarization,  which  exerts  an  effect  in  the 
opposite  direction  (cf.  p.  31). 

If  a  substance  soluble  in  water,  e.g.,  sugar,  lies  as  a  solid  on  the 
bottom  of  a  beaker  filled  with  water,  then  the  molecules  that  lie 
close  together  in  the  solid  substance  tend  to  distribute  themselves 
throughout  the  liquid,  or,  in  other  words,  the  substance  dissolves. 
This  tendency  of  the  solid  molecules  to  pass  into  the  liquid  may 
be  regarded  as  a  result  of  pressure  and  one  may  say  that  the 
solid  substance  possesses  a  solution  pressure.  If  sufficient  solid  is 
present,  then,  as  a  result  of  diffusion,  eventually  the  liquid  will 
reach  what  we  call  the  state  of  saturation.  The  liquid  then  contains 
an  equal  quantity  of  sugar  in  all  its  parts  and  at  the  prevailing 
temperature  it  will  not  take  up  any  more  sugar.  There  must 
also  be  some  cause  present  which  prevents  a  saturated  solution 
from  dissolving  any  more  of  the  solid  substance.  This  cause  is 
designated  as  the  osmotic  pressure  which  the  dissolved  molecules 
exert  in  the  solution.  The  dissolved  molecules,  like  the  molecules 
of  gas  over  an  evaporating  liquid,  exert  a  pressure  which  increases 
with  their  number;  when  the  osmotic  pressure  produced  by  a 
sufficiently  large  number  of  molecules  is  equal  to  the  solution 
pressure,  then  there  is  no  further  increase  in  the  concentration  of 
the  solution  and  the  solution  is  saturated.  There  is  then  an  equi- 
librium established  between  the  solution  pressure  and  the  osmotic 
pressure  and  there  is  just  as  much  tendency  for  molecules  to 
separate  out  from  the  solution,  owing  to  the  osmotic  pressure,  as 
there  is  for  solid  molecules  to  pass  into  solution  because  of  the 


24  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

solution  pressure.  In  the  saturated  solution,  i.e.,  in  the  solution 
which  is  in  equilibrium  with  the  solid,  the  solution  pressure  of  the 
solid  is  exactly  balanced  by  the  osmotic  pressure  which  the  solu- 
tion exerts.  This  process  of  dissolving  sugar  molecules  involves 
no  electrical  effects;  the  same  is  true  of  the  dissolving  of  salt 
because  when  the  molecules  ionize  an  equal  number  of  cations 
and  anions  are  formed  so  that  the  solution  remains  neutral. 

Like  the  readily  soluble  substances,  though  to  a  lesser  degree, 
the  metals  themselves  have  some  tendency,  when  in  contact  with 
a  liquid,  to  send  their  atoms  into  solution  in  the  form  of  ions. 
This  tendency  is  called  the  electrolytic  solution  pressure  of  the 
metals.  On  the  other  hand,  the  electrically  charged  ions  of  the 
metal  also  strive  to  pass  over  into  the  electrically  neutral  condi- 
tion and  the  cause  of  this  tendency  is  again  the  osmotic  pressure. 
The  electrolytic  solution  pressure  of  a  metal  and  the  osmotic 
pressure  of  its  ions  act  mutually  against  one  another  in  the  same 
way  as  solution  pressure  and  osmotic  pressure. 

The  transformation  of  the  atoms  of  a  metal  into  the  ionic 
condition,  and  conversely  the  transformation  of  an  ion  into  the 
atomic  condition,  is  closely  related  with  the  electrical  phenomena 
which  exist  between  the  metal  and  the  solution.  The  theory  of 
solutions  teaches  that  in  a  dilute  solution  of  zinc  sulphate  there 
are  present  an  equal  number  of  positively  charged  zinc  ions  and 
negatively  charged  sulphate  ions  and  the  solution  itself  is  elec- 
trically neutral.  If  a  piece  of  zinc  is  placed  in  the  solution,  then, 
as  a  result  of  its  electrolytic  solution  pressure,  the  metal  sends  some 
positively  charged  zinc  ions,  Zn++,  into  solution  and  these  ions  col- 
lect around  the  metal  and  form  a  positively  charged  layer  of  liquid. 

The  electrolytic  solution  pressure  has  a  definite  value  and  the 
osmotic  pressure  which  is  opposed  to  it  at  any  time  is  dependent 
upon  the  concentration  of  metal  ions  in 
solution.  If,  in  Fig.  6,  the  arrows  s.p.  o.P.  u.m.f. 
represent  the  value  and  direction  of  the 
electrolytic  solution  pressure  of  the  — > — >  +• 
metal,  and  if,  as  shown  in  the  figure,  it 
is  greater  than  the  osmotic  pressure, 

ions  pass  into  solution  and  the  solution    — •> >  4. 

itself  becomes  positively  charged  while 
the  corresponding  negative  charge  re- 
mains upon  the  metal.  Acting  upon 


FIG.  6. 


ELECTROMOTIVE  FORCE  OR  POTENTIAL       25 

the  ions  in  the  vicinity  of  the  metal  is  an  electrostatic  force  which 
seeks  to  drive  them  back  upon  the  metal.  There  is,  therefore, 
an  electromotive  force,  represented  by  the  dotted  arrows  E.m.f., 
added  to  the  osmotic  pressure  o.p.  and  it  increases  rapidly  with 
the  number  of  ions  that  pass  into  solution  (1  gram  equivalent 
carries  a  charge  of  96,500  coulombs)  and  when  the  sum  of  the 
osmotic  pressure  plus  the  electromotive  force  is  equal  to  the  elec- 
trolytic solution  pressure  s.p.,  equilibrium  results. 

The  conditions  are  somewhat  analogous  to  the  evaporation 
of  a  liquid;  the  liquid  will  evaporate  until  its  vapor  pressure  is 
balanced  by  the  pressure  of  the  gas  molecules.  When  any  metal 
is  placed  in  contact  with  a  solution  of  its  ions,  more  ions  will 
enter  the  solution  from  the  metal  if  the  electrolytic  solution  pres- 
sure of  the  metal  exceeds  the  osmotic  pressure  and  the  reverse 
phenomenon  will  take  place  in  case  the  osmotic  pressure  exceeds 
the  electrolytic  solution  pressure.  In  the  former  case,  the  metal 
becomes  negative  to  the  solution  and  in  the  latter  case  it  becomes 
positive  to  the  solution.  It  is  customary  to  assume,  according 
to  Helmholtz,  the  existence  of  an  electrical  double  layer  at  the 
junction  of  the  metal  and  the  solution.  In  the  case  of  the  zinc, 
referred  to  above,  this  consists  of  a  negatively-charged  layer  on 
the  metal  and  a  positively-charged  layer  in  the  solution  where 
it  is  in  contact  with  the  metal.  The  actual  existence  of  such 
a  double  layer  has  been  demonstrated  by  the  work  of  Palmer.* 
It  is  important  to  bear  in  mind  that  whether  ions  pass  from  the 
metal  into  the  solution  depends  not  only  upon  the  electrolytic 
solution  tension  of  the  metal  but  also  upon  the  osmotic  pressure 
already  prevailing  in  the  solution  and  this  osmotic  pressure 
is  proportional  to  the  concentration  of  the  solution.  The  potential 
difference  between  the  metal  and  the  solution  is  a  quantity  which 
can  be  easily  measured;  it  is  called  the  single  potential  or  the 
oxidation  potential  of  the  metal.  The  electrolytic  solution  pres- 
sure, on  the  other  hand,  cannot  be  measured  directly.  If,  how- 
ever, the  oxidation  potentials  of  the  metals  are  measured  against 
solutions  of  equivalent  concentrations,  these  potentials  will  bear 
the  same  relation  to  one  another  as  do  the  electrolytic  solution 
pressures.  Just  as  in  determining  the  height  of  any  object  it 
is  necessary  to  choose  arbitrarily  some  zero  level  from  which  to 
measure,  so  in  the  same  way  it  is  desirable  to  choose  an  arbitrary 
*  Z  phys.  Chem.,  25,  265;  28,  257;  36,  364. 


26  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

zero  for  measuring  the  oxidation  potentials.  A  number  of  standard 
cells  have  been  devised  for  this  purpose.  The  usual  standard  for 
comparison  is  that  of  the  hydrogen  electrode.  The  zero  potential, 
according  to  this  standard,  is  that  of  an  electrode  consisting  of  a 
strip  of  platinized  platinum,  half  in  pure  hydrogen  gas  and  half  in 
a  normal  solution  of  sulfuric  acid.  In  accordance  with  this  scale, 
the  oxidation  potentials  of  some  of  the  more  common  elements  and 
radicals  against  molal*  concentrations  of  their  ions  are  as  follows: 


Lithium +3.03 

Potassium 2 . 93 

Sodium 2.72 

Barium 2.8 

Strontium 2.7 

Calcium 2.6 

Magnesium, 

Manganese 1 . 08 

Zinc 0.77 

Iron 0.43 

Cadmium 0.42 

Cobalt 0.23 

Nickel 0.22 

Lead..  0.12 


Tin +0.10 

Hydrogen +0.00 

Copper —0.34 

Iodine —  0 . 52 

Silver -0.80 

Mercury' —0.86 

Bromine — 0 . 99 

Chlorine —1.35 

Gold -1.5 

Hydroxyl,  OH -1.68 

Sulfate,  SO4 -1.9 

Acetate,  C2H3O2 , .  -2.5 

Bisulfate,  HSO4 -2.6 


In  the  above  table  the  positive  sign  has  been  assigned  to  those 
elements  which  have  a  greater  tendency  to  form  ions  than  does 
hydrogen. 

The  simple  contact  of  a  metal  with  a  solution  always  results 
in  a  potential  difference  between  the  metal  and  solution  except 
when  the  osmotic  pressure  exactly  balances  the  electrolytic 
solution  pressure.  This,  however,  is  not  a  permanent  source 
of  electricity  because  a  state  of  equilibrium  is  reached  quickly 
by  either  entry  of  ions  into  the  solution  or  deposition  upon  the 
metal.  If,  however,  two  metals  of  different  oxidation  potential 
are  placed  in  contact  with  their  respective  solutions,  as  in  the 
Daniell  cell,  then  electric  charges  of  different  potentials  result 
and  if  the  two  metals  are  connected  outside  the  liquids  by  a 

*  A  molal  solution  contains  one  mole  per  liter  of  dissolved  substance.  The 
word  mole  signifies  a  molecular  weight  in  grams  and  when  ionization  takes 
place,  a  gram-ion  is  counted  as  a  mole.  Thus  one  mole  of  sodium  sulfate, 
Na2S04,  when  completely  ionized  furnishes  two  moles  of  sodium  ions  and  one 
mole  of  sulfate  ions.  With  respect  to  sodium  ions,  therefore,  the  molal  con- 
centration of  the  sodium  sulfate  solution  is  twice  as  large  as  it  is  with  respect 
to  sulfate  ions.  Inasmuch  as  the  extent  of  the  ionization  of  salts  in  solution 
is  not  positively  known,  it  is  extremely  unfortunate  that  the  values  in  the 
table  should  be  referred  to  molal  concentrations  of  the  ions. 


ELECTROMOTIVE  FORCE  OR  POTENTIAL       27 

wire  and  within  the  cell  the  two  solutions  are  also  in  contact  with 
one  another,  an  electric  current  flows  from  the  higher  potential 
to  the  lower.  Since  the  original  differences  in  potential  are  con- 
stantly re-established,  a  permanent  current  results.  The  positive 
to  negative  direction  of  the  current  is  from  zinc  to  copper  in 
the  solution  and  from  copper  to  zinc  in  the  wire.  In  the  above 
table  of  oxidation  potentials  many  physicists  place  negative 
signs  to  the  values  assigned  to  all  the  elements  above  hydrogen 
in  the  table  and  positive  signs  to  those  potentials  below  hydrogen. 
This  is  because  the  physicist  thinks  of  the  current  as  it  flows 
in  the  wire  from  the  Daniell  cell  and  regards  the  copper  as  positive 
to  the  zinc.  The  chemist,  on  the  other  hand,  has  his  attention 
fixed  on  the  chemical  changes  involved  and  traces  the  flow  of 
the  current  from  the  zinc  to  the  copper  in  the  cell.  The  chemist 
thinks  of  the  elements  at  the  top  of  the  series  as  the  more  positive 
elements  and,  to  him,  it  seems  logical  to  assign  positive  values 
to  the  oxidation  potentials  of  the  elements  which  are  most  easily 
oxidized.  According  to  this  view,  the  potential  is  positive  when 
the  charge  of  the  solution  is  positive  to  the  metal. 

Nernst,  who  suggested  the  above  explanation  of  the  origin  of 
the  electromotive  force  on  the  basis  of  osmotic  relations,  has 
worked  out  a  formula  for  computing  the  potential  difference  which 
exists  at  the  place  of  contact  of  a  metal  with  a  solution.  If  E 
denotes  this  potential  difference  expressed  in  volts,  R  the  gas 
constant  expressed  in  volts  X  coulombs,  F  the  electrochemical 
equivalent  or  quantity  of  electricity  required  to  deposit  one 
equivalent  weight  in  grams  of  any  substance,  n  the  valence  of  the 
metal  ions,  P  the  electrolytic  solution  pressure,  p  the  osmotic 
pressure,  and  T  the  absolute  temperature  of  the  solution,  the 
Nernst  formula  *  reads 

RT,      P 

E  = logg— . 

nv     &  p 

*  This  formula  is  derived  with  the  aid  of  integral  calculus.  If  one  gram-ioii 
of  a  metal  is  changed  from  the  electrolytic  solution  tension  P  to  the  osmotic 

Cp  Cpdp 

pressure  p,  the  osmotic  work  done  will  be  I     vdp  =  RT  I      — .      Integrating 

this  expression,  we  get  ^p  " p 

P 

Osmotic  work  =  RT  loge  — . 

The  corresponding  electrical  energy,  nFj&,  using  the  notation  as  above,  is 
equivalent  to  the  osmotic  work.  Hence 


28  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

If,  for  R  and  F,  we  substitute  their  numerical  values  and  divide  by 
0.4343  in  order  to  use  common  logarithms  instead  of  natural 
logarithms,  the  formula  becomes 

8.316  XT  P      0.0001983  T,     P 

E  =  0.4343  XnX96,540logp  =         —  ~  log  p  V°lt8' 

or,  in  round  numbers, 

0.0002  T,     P     u 

E  =  -  -    log  —  VOlts. 

n  p 

If  we  assume  that  the  ordinary  room  temperature  is  18°  C.,  the 
value  of  T  is  273  +  18  =  291  and  the  formula  becomes 

0.058,      P 
El8°=  —-log -volts. 

The  total  electromotive  force  of  a  galvanic  element  is  equal  to 
the  difference  between  the  single  potentials  and  can  be  measured 
by  inserting  a  voltmeter  of  high  resistance  between  the  poles  of 
the  element.  In  the  Daniell  cell,  when  the  zinc  sulphate  and 
copper  sulphate  solutions  are  both  of  normal  concentration,  the 
total  electromotive  force  of  the  element  is  the  difference  between 
the  oxidation  potential  of  zinc  (-f  0.76  volt)  and  that  of  copper 
(-  0.34  volt)  =  1.10  volts. 

In  the  Nernst  formula,  the  osmotic  pressure,  p,  is  determined 
by  the  concentration  of  the  solution.  If  the  concentration  of  a 
solution  is  decreased  ten  fold,  the  osmotic  pressure  in  round 

numbers  is  also  decreased  ten  fold  and  the  value  of  the  expres- 

p 
sion  log  —  ( =  log  P  —  log  p)  is  increased  1  unit.     If  the  metal 

is  univalent,  the  oxidation  potential  will  be  increased  0.058  volt, 
or  0.029  volt  if  the  metal  is  bivalent.  The  table  on  page  26 
shows  that  the  oxidation  potential  of  copper  is  —  0.34  volt  against 
a  normal  solution  bivalent  copper  ions.  If  the  concentration  of 
the  cupric  solution  is  reduced  to  1  X  10  ~12  normal,  then  the 
oxidation  of  the  copper  will  be  practically  0.01.  If  the  solution 
of  cupric  ions  is  more  concentrated  than  this  extremely  low  value, 
the  oxidation  potential  of  the  copper  will  be  less  than  that  of 
hydrogen  in  the  normal  electrode. 

Although  in  electro-analysis  less  depends  \ipon  the  production 
of  the  current  than  upon  its  consumption,  the  above  discussion 
will  help  one  to  understand  the  changes  which  take  place  in  the 
deposition  of  metals;  if  a  difference  in  potential  is  caused  by  the 
process  of  solution,  which  corresponds  to  the  accomplishment  of 


ELECTROMOTIVE  FORCE  OR  POTENTIAL       29 

work,  similarly,  in  the  reverse  process  of  depositing  a  metal,  work 
must  be  expended  in  overcoming  a  difference  in  potential.  It  was 
mentioned  on  page  23,  however,  that  potential  differences  arise  in 
the  electrolytic  cell  and  work  in  opposition  to  these  electromotive 
forces  we  have  been  discussing;  this  will  be  explained  soon. 

If  two  platinum  electrodes  are  dipped  into  a  solution  of  a  metal 
salt  and  the  electrodes  are  connected  through  a  voltmeter,  the 
instrument  will  not  show  any  current.  There  are  two  reasons 
why  no  electromotive  force  results:  first,  because  there  is  no 
reaction  taking  place  at  the  unattacked  electrodes  and  second,  if 
the  electrodes  were  attacked  the  reactions  would  be  the  same  at 
each.  If,  however,  the  electrodes  are  connected  with  the  poles 
of  a  source  of  electricity,  then  one  will  be  positively  charged  and 
serve  as  anode  while  the  other  will  be  negatively  charged  and  act 
as  cathode.  The  charges  on  the  ions  will  then  become  neutralized 
by  the  charges  on  the  electrodes.  The  positive  charge  on  the  anode 
neutralizes  the  negative  charge  of  the  anions  which  are  thereby 
changed  to  the  atomic  condition.  Similarly,  the  negative  charge 
on  the  cathode  neutralizes  the  positive  charge  of  the  cations, 
which  are  likewise  changed  into  the  atomic  condition  and  (in  most 
cases)  are  deposited  as  such  upon  the  cathode.  This  is  the  quali- 
tative side  of  the  process  of  electrolysis.  The  question  now 
arises  —  What  are  the  quantitative  relations?  Does  an  electro- 
motive force  produced  at  the  electrodes  cause  in  the  electrolyte 
a  current  strength  which  corresponds  to  the  resistance  of  the 
electrolyte?  In  other  words,  Does  the  process  follow  Ohm's  law 
exactly  as  in  the  case  of  a  metallic  conductor?  From  the  experi- 
ment described  below  one  would  at  first  sight  conclude  that  this 
is  not  the  case,  but  the  subsequent  explanation  will  show  that 
Ohm's  law  is  applicable  in  all  cases.  Imagine  two  platinum 
electrodes,  each  having  a  surface  of  1  square  centimeter,  placed 
1  centimeter  apart  in  dilute  sulphuric  acid,  so  that  the  volume 
of  liquid  between  the  electrodes  corresponds  exactly  to  1  cubic 
centimeter.  With  5  per  cent  sulphuric  acid  this  cube  would  have 
approximately  5  ohms  resistance  (cf.  p.  20).  If  we  send  through 
this  resistance  a  current  of  such  a  strength  that  the  electro- 
motive force  is  0.5  volt  between  the  electrodes,  then,  according  to 

Ohm's   law,  the   current   strength  would   be    -^-  =  0.1  ampere 

o 

provided  the  same  conditions   hold  as  in  metallic  conductors. 


30  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

If  the  electromotive  force  is  increased  to  1  volt,  then  the  cur- 
rent strength  will  be  doubled  and  become  0.2  ampere.  If  these 
values  are  plotted  ;Fig.  7)  with  the  abcissas  representing  volts 
and  the  ordinates  amperes,  the  curve  representing  the  ratio  of 
volts  to  amperes  will  take  the  course  of  the  straight  line  OAB. 
If,  however,  the  actual  values  obtained  by  experiment  are  plotted 
on  the  diagram,  starting  with  the  voltage  at  0,  gradually  strength- 
ening it  and  measuring  the  current  strength  at  0.5  volt,  1  volt, 
etc.,  then  the  points  A',  B',  C'  will  be  obtained,  and  by  connecting 


Amp. 
0.3 


FIG.  7. 

them  the  curve  will  show  that  the  current  strength  increases  much 
more  slowly  than  would  be  expected  from  Ohm's  law.  Suddenly, 
at  the  point  C',  which  corresponds  to  1.67  volts,,  the  curve  changes 
its  direction  and  from  this  point  the  current  strength  increases 
more  rapidly,  as  the  line  C'D'  shows. 

The  diminution  of  the  current-strength,  and  the  apparent  devia- 

IP 

tion  from  Ohm's  law,  I  =  - ,  could  be  accounted  for  by  an  increased 

R 

resistance  R,  or  by  a  diminished  electromotive  force  E.  As  regards 
the  resistance,  it  remains  practically  constant.  On  the  other  hand, 
it  is  easy  to  demonstrate  that  an  electromotive  force  results 
between  the  electrodes  which  acts  against  that  of  the  applied 
current.  If,  after  the  current  has  passed  for  a  short  time,  the 
connection  with  the  source  of  current  is  broken,  and  the  circuit 
is  closed  again  with  a  galvanometer  or  voltmeter  inserted  be- 
tween the  electrodes,  a  current  flowing  from  the  cathode  to  the 
anode  through  the  acid  will  be  detected  and  it  will  have  the 
opposite  direction  to  that  of  the  original  current.  This  current 
persists  only  a  short  time  and  the  pointer  of  the  voltmeter  soon 
falls  back  to  the  zero  reading. 

Such  a  current  is  called  a  polarization  current  and  its  formation 
depends  upon  the  nature  of  the  substances  set  free  at  the  elec- 
trodes. In  the  above  case,  the  original  current  caused  hydrogen 


ELECTROMOTIVE  FORCE  OR  POTENTIAL       31 

to  be  evolved  at  the  cathode  and  oxygen  at  the  anode,  i.e.,  two 
gases.  If  the  solution  of  a  salt  such  as  cupric  chloride  were  elec- 
trolyzed,  then  copper  would  be  deposited  upon  the  cathode  and 
chlorine  set  free  at  the  anode,  i.e.,  a  metal  and  a  gas.  This  is  the 
most  common  case  in  electro-analysis.  In  all  cases,  the  original, 
unattacked  electrodes  become  coated  with  foreign  substances  so 
that  they  behave  like  two  different  metals  which  are  placed  in 
contact  with  a  solution  and  are  striving  to  send  ions  into  it  (cf. 
p.  24);  in  place  of  the  original  cell  Pt  |  CuCl2  1  Pt  a  new  combina- 
tion Cu  |  CuCl2  1  C12  has  been  formed  and  this  represents  an  active 
galvanic  element.  This  is  the  opposing  electromotive  force  in 
the  cell  which  was  referred  to  on  page  23. 

The  potential  of  the  polarization  current,  or  the  electromotive 
force  of  polarization,  can  be  measured  in  several  different  ways 
(see  below).  If  we  designate  the  polarization  potential  as  E2, 
the  potential  of  the  original  current  as  EI,  and  the  total  resistance 
as  R,  then  the  equation  representing  Ohm's  law  for  an  electrolyte  is 

EI  —  E2 

i  =  -  or   EI  =  IR  +  E2. 


If,  starting  from  the  value  zero,  EI  is  made  to  increase  slowly, 
measurements  will  show  that  at  first  E2  is  nearly  equal  to  EI,  but 
as  the  value  of  EI  increases,  that  of  E2  increases  much  more  slowly, 
without,  however,  reaching  a  maximum. 

The  experiment  with  sulphuric  acid,  described  on  page  29  and 
illustrated  by  Fig.  7,  shows  that  the  electrolysis  of  the  acid  should 
not  take  place  with  a  potential  of  less  than  1.67  volts;  the 
current  strength  with  lower  voltage  currents  is  so  slight  that 
practically  no  current  passes  through  the  solution  and  conse- 
quently there  is  no  appreciable  decomposition  of  the  electrolyte. 
All  other  acids  behave  like  sulphuric  acid  and  the  same  is  true  of 
solutions  containing  bases  or  salts,  especially  salts  of  the  heavy 
metals,  with  which  we  are  chiefly  concerned  in  electro-analysis. 
There  is  for  every  electrolyte  a  certain  value  which  must  be  given 
to  the  potential  of  the  current  in  order  to  effect  a  permanent 
decomposition  of  the  electrolyte.  This  value  has  been  called  the 
decomposition  potential  and  LeBlanc  has  determined  it  for  many 
electrolytes.  The  following  table  gives  the  decomposition  poten- 
tials of  a  few  salts  in  normal  solutions:  * 

*  These  decomposition  potentials  were  measured  by  Le  Blanc  in  1891. 
The  values  given  are  those  for  the  easiest  possible  decomposition.  Thus,  in 


32  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

ZnSO4       =  2.35  volts  Cd(NO3)2  =  1.98  volts 

ZnBr2        =  1.80  volts  CdSO4  =  2.03  volts 

NiSO4       =  2.09  volts  CdCl2  =  1.88  volts 

NiCl2        =  1.85  volts  CoS04  =  1.92  volts 

Pb(NO3)2  =  1.52  volts  CoCl2  =  1.78  volts 
AgNO3      =  0.70  volt 

The  decomposition  potentials,  which  are  to  be  regarded  as  the 
constant  minimum  of  the  polarization  potential  of  a  solution, 
vary,  therefore,  with  different  metals;  the  values  are  not  far 
apart  for  the  sulphates  and  nitrates  of  the  same  metal,  as  is  shown 
in  the  table  for  the  corresponding  cadmium  salts. 

The  decomposition  potential  Ed  consists  of  the  potential  EC 
required  at  the  cathode  for  the  deposition  of  the  metal  and  of 
the  potential  EO  required  at  the  anode  to  liberate  oxygen  or  other 
element.  Thus 

Ed  =  Ec  +  Ea. 

Since,  however,  the  decomposition  potential  denotes  the  mini- 
mum potential  that  is  required  to  cause  an  electric  current  to 
pass  through  an  electrolyte,  or,  in  other  words,  it  represents  the 
electromotive  force  opposed  to  the  passage  of  the  current  that 
causes  electrolysis,  it  is  obvious  that  the  main  current  must  have 
a  greater  potential  if  a  continuous  flow  or  a  suitable  current 
strength  is  to  be  maintained. 

The  excess  potential  eQ  is  dependent  upon  the  Ohm's  resistance 
B  of  the  electrolyte  and  the  desired  current  strength;  according 
to  Ohm's  law 

EO 

I  =  -     or   E0  =  IK. 
K 

The  potential  E  which  the  voltmeter  shows  when  placed  in 
circuit  between  the  electrodes  during  an  electrolysis  experiment  is 

E  =  Ed  +  E0  =  Ec  +  Ea  +  IR, 

and  from  this  the  current  strength  i  can  be  computed: 

!  =  E  -  (Ec  +  Eq) 
R 

This  formula,  therefore,  expresses  Ohm's  law  as  it  applies  to 
electrolytes  (cf.  p.  29). 

the  case  of  zinc  sulphate,  the  values  are  those  obtained  for  the  deposition  of 
zinc  on  the  cathode  and  liberation  of  oxygen  (not  SO4)  at  the  anode. 


ELECTROMOTIVE  FORCE  OR  POTENTIAL       33 

The  same  formula  can  also  be  used  for  determining  the  de- 
composition potential  Ed  of  an  '  electrolyte.  From  the  equation 

E-CB.  +  BJ     orI=£^Ei 

R  R 

it  follows  that 

Ed  =  E  —  IR. 

In  this  last  formula,  E  is  the  potential  of  the  bath  as  shown  by 
the  voltmeter,  i  is  the  current  strength  shown  by  the  ammeter, 
and  R  is  the  resistance  of  the  electrolyte  which  can  be  determined 
as  described  on  page  19. 

The  resistance  value  R  can  be  eliminated  from  the  formula 
by  making  two  observations  with  changed  current  strength;  for 
if,  in  a  second  observation,  it  is  found  that 


then  from  the  equations  (I)  and  (II)  we  find  that 

IlE  —  IEi 

Ed  =  -  -- 

II  -I 

If  in  this  way  the  value  of  Ed  is  known,  then  the  value  for  R 
can  be  computed  from  eith'er  equation  (I)  or  equation  (II). 

The  decomposition  potential  of  any  given  solution  can  be 
measured  by  placing  two  platinum  wires  in  the  solution  to  serve 
as  electrodes  and  allowing  the  current  to  increase  gradually  in 
strength  until  a  constant  reading  is  obtained  with  a  sensitive 
galvanometer  placed  in  the  circuit. 

These  decomposition  potentials  are  important  for  two  reasons; 
first,  because  they  represent  the  minimum  electromotive  force 
that  is  required  to  effect  the  deposition  of  a  metal,  and  second, 
because  they  show  how  certain  metals  can  be  separated  quan- 
titatively from  one  another  by  varying  the  potential.  For 
example,  if  a  solution  contains  silver  nitrate  and  zinc  sulphate, 
the  table  shows  that  it  is  possible  to  deposit  the  silver  with  a 
current  at  0.7  volt  while  the  zinc  will  not  be  deposited  until  the 
electromotive  force  is  raised  to  2.35  volts.  Thus,  by  keeping 
the  potential  above  0.7  volt  and  below  2.35  volts,  the  silver  can 
be  deposited  quantitatively  and  then,  by  raising  the  potential 
above  2.35  volts,  the  zinc  can  be  deposited. 

As  already  mentioned,  Kiliani  was  the  first  to  recognize  the 
importance  of  the  potential  of  the  current  in  electrolytic  separa- 


34  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

tions.  Profiting  by  the  studies  of  LeBlanc,  Freudenberg,  working 
in  Ostwald's  laboratory,  was  able  subsequently  to  study  such 
relations  more  accurately.  Use  was  made  of  these  studies  in 
methods  described  for  determining  and  separating  certain  metals. 
The  fact  that  this  principle  of  effecting  electrolytic  separations 
by  graded  potentials  is  not  applicable  to  all  separations  will  be 
shown  later. 

Ordinarily,  the  electromotive  force  of  the  cell  is  measured  by 
means  of  a  voltmeter  which  shows,  in  volts  and  fractions  thereof, 
the  drop  in  potential  that  takes  place  in  the  electrolyte  between 
the  two  electrodes.  The  voltmeter  is  in  reality  an  ammeter  with 
a  large  internal  resistance,  whereas  the  ammeter,  which  serves  to 
measure  the  current  strength,  must  have  as  little  resistance  as 


possible  because  it  is  placed  in  the  circuit  and  must  not  diminish 
the  current  strength  appreciably.  The  two  wires  leading  to  the 
voltmeter  are  each  attached  to  one  of  the  electrodes;  the  instru- 
ment is  then  connected  as  a  shunt  to  the  circuit  and  in  parallel 
with  the  electrolyte.  The  resistance  of  the  voltmeter  is  so  great 
that  nearly  all  of  the  current  continues  to  pass  through  the  cell  with 
a  practically  unchanged  electromotive  force  and  only  an  inappre- 
ciably small  fraction  of  the  whole  current  passes  through  the 
voltmeter.  If  the  resistance  of  the  voltmeter  were  much  less,  then 
too  large  a  fraction  of  the  whole  current  would  pass  through  the 
instrument  and  as  a  result  less  current  would  pass  through  the 
electrolyte  so  that  the  drop  in  potential  between  the  electrodes 
would  be  noticeably  less  than  when  the  voltmeter  was  disconnected. 
Let  us  assume  that  an  electrolytic  cell  Z  is  placed  between  the 
points  A  and  B  (Fig.  8)  in  a  circuit  and  that  it  is  desired  to  meas- 
ure the  electromotive  force  E  between  A  and  B  by  means  of  the 
voltmeter  V.  The  current  strength  i,  which  prevails  in  the 
circuit  AZB  before  the  insertion  of  the  voltmeter,  is  changed,  after 


ELECTROMOTIVE  FORCE  OR  POTENTIAL       35 

the  introduction  of  the  voltmeter,  into  the  two  current  strengths, 
i  in  AZB  and  i'  in  AVB;  thus  i  =  i  +  i' *  If,  now,  r  and  r'  are 
the  respective  resistances  in  the  two  branch  circuits,  then 


E  ,    .,      E 

i  =  -  and  i  =  — 

r  r 

and  consequently 


/  7 


r  +  r' 

rrr 
Since  ,  is  smaller  than  r,  it  is  obvious  that  the  potential  E  will 

always  become  smaller  as  a  result  of  introducing  the  shunt,  but 
it  remains  at  approximately  its  original  value  when  r'  is  very 
large.  In  the  latter  case,  i'  becomes  very  small  and  i  retains 
approximately  the  original  value  i. 

The  voltmeter  serves,  as  mentioned,  to  measure  the  difference 
in  potential  between  any  two  points  in  the  circuit,  usually  the 
electrodes  of  the  cell.  In  electro-analysis,  however,  it  is  often 
necessary  to  determine  a  single  potential  and  the  following  dis- 
cussion will  show  how  this  can  be  done. 

As  stated  on  page  24,  there  result  at  the  place  of  contact  be- 
tween metal  and  liquid  in  a  cell  certain  differences  in  potential 
which  are  independent  of  one  another  and  these  differences  are 
the  cause  of  the  electromotive  force  of  the  cell. 

These  single  potentials  are  also  called  potential  drops.  In  all 
parts  of  the  circuit  between  the  electrodes,  outside  as  well  as 
within  the  element,  the  potential  falls  continuously  if  measured 
between  any  point  and  the  point  with  the  lowest  potential.  At 
the  place  of  contact  of  metal  and  liquid,  however,  there  is  a  sudden 
change  in  the  potential,  or  drop. 

Such  drops  in  potential  also  result  at  the  platinum  electrodes 
in  an  electrolyte  when  the  electromotive  force  of  the  primary 
current  has  reached  the  decomposition  value,  and  the  electro- 
motive force  of  the  polarization  current,  just  as  that  of  the  ordi- 

*  Strictly  speaking  the  current  strength  i  is  increased  slightly  by  placing 
the  voltmeter  in  the  circuit  because  the  resistance  of  the  entire  system  is 
slightly  diminished  by  giving  the  current  another  path  to  traverse;  this 
increase  in  current  is  so  slight  that  it  need  not  be  taken  into  consideration 
in  the  above  explanation. 


36  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

nary  galvanic  element,  may  be  regarded  as  the  difference  between 
two  independent  potential  drops,  one  at  the  cathode  and  one  at 
the  anode  (p.  25).  When  the  decomposition  potential  is  exceeded 
the  metal  of  the  electrolyte  appears  upon  the  cathode  so  that  at 
the  decomposition  point  of  the  solution  the  potential  drop  at  the 
cathode  must  be  equal  to  the  difference  in  potential  which  the  metal 
deposited  on  the  cathode  shows  independently  toward  the  solution 
(LeBlanc). 

The  knowledge  of  the  single-potential  differences  has  become 
of  great  importance  in  electro-analysis.  Formerly,  in  depositing 
a  metal  from  a  solution,  one  was  content  to  know  the  total  differ- 
ence in  potential  between  the  two  electrodes;  the  bath  was  given 
a  somewhat  greater  voltage  than  corresponds  to  the  decomposi- 
tion value  (p.  32).  It  was  believed  that  two  metals  could  be 
separated,  if,  as  in  the  example  on  page  33,  the  voltage  was  kept 
between  the  decomposition  potentials  of  the  two  metals  and  then, 
after  one  metal  was  deposited  completely,  the  voltage  was  raised 
above  the  decomposition  value  of  the  second  metal.  From  the 
studies  of  LeBlanc,  however,  a  much  more  accurate  rule  to  follow 
has  been  obtained;  it  is  important  that  the  cathode,  upon  which 
the  metal  is  to  be  deposited,  shall  be  brought  to  at  least  the 
potential  which  the  metal  itself  shows  toward  the  solution.  Many 
failures  attendant  upon  former  analytical  directions  can  be  traced 
to  the  nonobservance  of  this  rule.  That  it  is  not  always  sufficient 
to  measure  the  total  voltage  and  to  regulate  it  in  the  deposition 
of  a  metal,  or  in  the  separation  of  two  metals  by  electrolysis,  is 
obvious  when  one  remembers  that  the  total  voltage  is  the  sum 
of  the  potential  drops  at  the  cathode  and  at  the  anode.  Since 
these  two  drops  in  potential  are  independent  of  one  another  and 
since  they  change  during  the  progress  of  the  analysis,  owing  to 
the  diminishing  concentration  of  the  salt  in  solution,  and  in  fact 
these  changes  are  independent  of  one  another,  it  may  happen  in 
a  simple  deposition  of  a  metal  that  the  cathode  potential  may 
change  in  a  manner  unfavorable  for  the  complete  deposition,  and 
at  the  same  time  the  potential  at  the  anode  may  change  inde- 
pendently in  such  a  way  that  the  total  voltage  will  remain  about 
the  same  as  at  first.  Thus  in  the  deposition  of  a  metal  the  point 
may  be  reached  where  the  unfavorable  evolution  of  hydrogen 
takes  place  before  all  the  metal  is  deposited.  In  a  separation  of 
two  metals,  the  cathode  potential  may  reach  a  value  which  per- 


ELECTROMOTIVE  FORCE  OR  POTENTIAL       37 

mils  the  deposition  of  the  second  metal  so  that  a  quantitative 
separation  becomes  impossible.* 

It  may  be  well,  here,  to  explain  the  principles  underlying 
the  measurement  of  these  single  potentials.  Since,  however,  the 
measurement  of  a  single  potential  almost  always  depends  upon  the 
measurement  of  a  potential  difference  between  two  single  poten- 
tials, of  which  one  is  known,  the  first  thing  to  describe  is  how  a 
potential  difference  is  measured.  The  ordinary  method  for 
doing  this  is  PoggendorfTs  compensation  method. 

Just  as  the  measurement  of  resistances  is  based  upon  the  com- 
parison of  the  resistance  to  be  measured  with  a  known  resistance 
in  the  Wheatstone  bridge  (cf.  p.  18),  so,  for  the  measurement 
of  potential  differences,  or  electromotive  forces,  a  source  of  current 
is  used  which  possesses  a  known  and  unchangeable  electromotive 
force.  Such  a  source  of  current  is  the  so-called  normal  element, 
e.g.,  the  Weston  element.  One  pole  of  this  element  consists  of 
mercury  in  contact  with  mercurous  sulphate  and  the  other  of 
cadmium  in  contact  with  cadmium  sulphate.  The  salts  of  the 
metals  are  not  contained,  however,  in  solutions  of  varying  con- 
centrations as  in  ordinary  elements  but  are  present  in  the  form  of 
solid  salts  in  contact  with  saturated  solutions,  whereby  the  action 
of  the  element  becomes  constant. f  The  chemical  reactions  which 
cause  the  current  are  metallic  cadmium  going  into  solution  at  the 
cadmium  pole  and  mercury  depositing  from  mercurous  sulphate 
at  the  mercury  pole,  just  as  in  the  Daniell  element  zinc  is  dissolved 
and  copper  deposited.  The  current  in  this  normal  element  flows 
within  the  cell  from  the  cadmium  to  the  mercury  and  outside  the 
cell  from  the  mercury  to  the  cadmium.  The  electromotive  force 
of  the  Weston  element  is  1.0186  -  0.00038  (t  -  20)  volts;  t  is  the 
temperature  of  the  element  when  in  use,  and  is  usually  the  labora- 

*  Similar  relations  prevail  in  ordinary  analytical  chemistry.  If,  for  example, 
some  silver  nitrate  solution  is  added  gradually  to  a  solution  containing  both 
sodium  chloride  and  potassium  chromate,  at  first  silver  chloride  will  be  pre- 
cipitated, and  it  is  only  when  an  excess  of  silver  solution  is  present  that  the 
chromate  is  acted  upon  and  silver  chromate  precipitates. 

t  The  solution  always  remains  saturated  with  both  salts;  in  this  way  a 
constant  concentration  of  the  salts,  upon  which  the  electromotive  force  de- 
pends according  to  Nernst's  formula,  is  maintained.  Otherwise  the  utilization 
of  the  current  from  the  cell  would  result  in  increasing  the  concentration  of  the 
zinc  sulphate  and  diminishing  that  of  the  mercurous  sulphate,  and  the  electro- 
motive force  of  the  element  would  then  vary. 


38  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

tory  temperature.  The  correction  member  of  the  formula  shows 
that  the  electromotive  force  of  the  element  is  only  very  slightly 
influenced  by  changes  in  temperature. 

The  measurement  of  the  electromotive  force  is  carried  out  as 
follows:  A  circuit  is  established  consisting  of  a  storage  cell,  or 
accumulator,  A  (Fig.  9)  and  a  wire  BC  of  uniform  cross  section 
and  high  resistance;  there  then  prevails  in  the  wire  between  one 
end  of  it,  B,  and  any  other  A 

point,  D,  D'  or  C,  a  certain  ^^ If 

drop  in  potential  which  can 
be  measured  by  means  of 
the  normal  element.  To  do 
this,  another  electric  circuit 
BGED,  containing  the  nor- 
mal element  E  and  a  sensitive 
galvanometer  G  (see  p.  43), 
is  connected  at  B  in  such  a 

way  that  the  current  in  this  second  circuit  flows  in  the  op- 
posite direction  e  to  that  of  the  other  current  which  has  the 
direction  a.  Then  by  moving  the  sliding  contact  D  along  the 
wire  AC,  a  point  will  be  found  for  which  the  galvanometer  G 
shows  the  passage  of  no  current.  Then  the  drop  in  potential 
between  the  points  B  and  D  is  equal  to  the  electromotive  force 
of  the  normal  element  E,  namely  1.0186  volts  at  20°. 

If,  now,  in  place  of  the  normal  element  E  an  unknown  electro- 
motive force  EX  is  introduced  and  the  sliding  contact  is  moved  to 
a  point  D'  for  which  the  galvanometer  reading  is  zero,  then  the 
unknown  electromotive  force  EZ  is  to  that  of  the  normal  element 
1.0186  as  the  length  of  wire  BD  is  to  the  length  BD'.  These 
lengths  are  known  in  millimeters,  and 

DTV 

E,  =  1.0186 -^ 

is  the  desired  electromotive  force. 

There  is,  therefore,  no  difficulty  in  determining  a  difference  of 
potential;  i.e.,  with  any  given  element  whose  cathode  potential 
is  EC  and  whose  anode  potential,  is  Ea,  it  is  easy  to  measure  the 
value  EC—  Ea  in  volts.  This,  however,  gives  us  no  information 
concerning  the  two  single  potentials  EC  and  EO.  If,  on  the  other 
hand,  we  are  able  to  prepare  an  element  in  which  one  of  the  two 
single  potentials  has  the  value  zero,  then  evidently  the  deter- 


ELECTROMOTIVE  FORCE  OR  POTENTIAL       39 

mination  of  the  electromotive  force  of  this  element  will  give 
directly  the  value  of  the  other  single  potential. 

Such  an  element  is  obtained  by  placing  some  metallic  mercury 
in  a  glass  vessel,  covering  it  with  dilute  sulphuric  acid  and  intro- 
ducing into  the  acid,  from  above,  a  capillary  tube  through  which 
mercury  flows  in  fine  drops.  If  the  mercury  resting  at  the  bottom 
of  the  vessel  is  connected  by  a  wire  with  the  mercury  in  the  tube 
from  which  the  mercury  drops,  a  current  can  be  detected  which 
flows  from  the  mercury  at  the  bottom  to  the  mercury  in  the 
dropping  tube.  The  electromotive  force  of  such  an  element  can 
be  measured,  and  since  Helmholtz  has  concluded  from  theoretical 
considerations  that  the  upper  electrode  (drop  electrode)  possesses 
the  potential  zero,*  it  is  evident  that  the  electromotive  force  of 
this  element  represents  the  potential  of  the  mercury  resting  at 
the  bottom  of  the  vessel. 

This  is  not  the  place  to  discuss  the  theory  of  such  an  element; 
it  may  be  mentioned  merely  that  we  are  dealing  here  with  one 
of  the  so-called  concentration  cells.  We  have  seen  (p.  26)  that 
different  metals  placed  as  electrodes  in  an  electrolyte  assume 
different  potentials  and  that  consequently  a  current  will  pass 
between  two  such  electrodes  if  the  outside  ends  are  connected  by 
a  wire.  We  have  also  seen  (p.  29)  that  two  strips  of  one  and  the 
same  metal  placed  in  the  same  electrolyte  will  show  no  potential 
differences.  If,  however,  two  electrodes  of  the  same  metal  are 
placed  opposite  to  one  another  in  an  electrolyte  •  in  which  the 
concentration  at  one  pole  is  greater  than  it  is  at  the  other,  then  a 
difference  in  potential  results;  the  metal  in  contact  with  the  more 
dilute  solution  is  ionized,  or  dissolved,  and  at  the  opposite  elec- 
trode these  ions  are  discharged  or  deposited.  Thus,  on  closing 
such  a  circuit,  a  current  flows  inside  the  cell  from  the  lower  con- 
centration to  the  higher  concentration.  Nernst  has  called  such 
an  arrangement  a  concentration  cell. 

When  mercury  is  in  contact  with  dilute  sulphuric  acid  it  can  be 
assumed  that  slight  traces  of  mercurous  ions  pass  into  solution; 
these  ions  result  either  from  the  presence  of  slight  traces  of  mer- 
curous oxide  adhering  to  the  metal,  which  dissolve  in  the  acid,  or 
the  oxide  may  be  formed  on  the  mercury  from  dissolved  oxygen 
that  is  present  in  the  acid  used.  The  potential  of  the  still 
mercury  in  the  element  with  the  drop  electrode  is  due  to  the 
*  Nernst  has  questioned  the  correctness  of  this  assumption. 


40 


QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 


contact  of  metallic  mercury  with  the  solution  of  its  ions;  and,  in 
fact,  this  potential  is  negative,  because,  as  was  shown  on  page  25, 
the  solution  pressure  of  the  noble  metals  is  very  slight,  being  less 
than  the  osmotic  pressure  of  the  corresponding  ions. 

The  single  potential  of  the  mercury  toward  the  solution  of 
mercurous  ions,  measured  as  described  above,  remains  the  same 
if,  as  indicated  on  page  39,  we  prepare  an  element  in  which  one  of 
the  electrodes  consists  of  mercury  in  contact  with  mercurous  ions 
while  the  other  electrode  consists  of  any  given  metal  in  another 
electrolyte.  The  electromotive  force  of  such  a  cell  can  be  measured 
by  the  compensation  method  (see  p.  38),  and  it  is  only  necessary 
to  deduct  from  the  number  of  volts  thus  found  the  known  potential 
of  the  mercury  electrode  in  order  to  obtain  the  potential  of  the 
other  electrode. 

This  is  a  rough  outline  of  the  principles  involved  in  the  measure- 
ment of  single  potentials  and  it  remains  only  to  mention  how 
the  measurements  are  made  c 
in  practice.  It  was  stated 
above  that  we  have  to  pre- 
pare a  cell  from  the  mercury 
electrode  and  the  electrode 
whose  potential  we  desire  to 
measure.  This  must  be  done 
without  disturbing  the  elec- 
tro-analysis in  the  interest  of 
which  such  a  measurement  is 
to  be  made  and  to  accomplish 
this  a  so-called  auxiliary  or 
normal  electrode  is  used;  it  is 
connected,  as  described  be- 
low, with  the  electrode  whose 
potential  is  to  be  measured. 
The  glass  vessel  shown  in  Fig. 
10,  at  one  half  its  true  size, 
contains  at  the  bottom  a 
layer  of  mercury  and  the  latter  is  connected  with  a  binding  post 
by  means  of  some  platinum  wire  fused  in  the  glass.  The  mercury 
is  covered  with  a  layer  of  mercurous  sulphate  M,  and  the  vessel 
is  nearly  rilled  with  2  N-sulphuric  acid,  saturated  with  mercurous 
sulphate.  The  glass  tube  fused  on  the  side  carries  in  the  middle 


FIG.  10. 


ELECTROMOTIVE  FORCE  OR  POTENTIAL       41 

of  the  horizontal  arm  a  stopcock  H  with  a  funnel  fused  to  it.  In 
the  drawing  the  cross  section  of  the  stopcock  is  drawn  to  show  the 
right-angled  boring  that  it  contains,  by  means  of  which  the  funnel 
can  be  connected  either  with  the  half  A  or  the  half  B,  or,  when  the 
stopcock  is  in  the  position  shown  in  the  drawing,  all  connections  are 
broken.  If,  at  the  start,  connection  is  made  with  A,  then,  on 
opening  the  pinchcock  and  blowing  in  air  at  c,  the  acid  can  be 
driven  over  until  it  reaches  the  stopcock  H,  when  the  connection 
is  broken  by  turning  the  stopcock  and  thus  the  half  of  the  tube 
marked  A  is  filled  once  for  all  with  the  acid. 

If  the  funnel  is  next  connected  with  the  half  of  the  tube  B,  then 
this  half  of  the  tube  can  be  filled  with  any  desired  solution.  The 
tube  B  ends  in  a  capillary  which  is  bent  up  and  down  a  number 
of  times  and  finally  points  upward.  The  shape  of  this  capillary 
is  shown  in  Fig.  10  by  a  separate  drawing;  in  reality  the  plane 
in  which  the  bendings  lie  is  perpendicular  to  the  plane  of  the 
paper.  This  shape  of  the  end  of  the  tube  is  devised  to  prevent, 
as  far  as  possible,  the  mixing  of  the  electrolyte  with  the  contents 
of  the  tube  B.  The  liquid  chosen  is  one  that  is  indifferent  toward 
the  electrolyte  and  ordinarily  consists  of  a  solution  of  sodium 
sulphate. 

This  arrangement  represents,  therefore,  the  bottom  layer  of  mer- 
cury, as  described  in  the  element  with  drop  electrode,  in  contact 
with  a  solution  containing  a  very  few  mercurous  ions,  as  mercurous 
sulphate  is  only  slightly  soluble.  The  sodium  sulphate  solution 
serves  merely  as  an  indifferent  conducting  liquid,  for  if  the  end 
of  the  capillary  tubing  is  dipped  into  an  electrolyte,  then  the 
sodium-sulphate  solution  serves  to  make  connection  between  the 
electrolyte  and  the  sulphuric  acid  in  A  because  the  sulphuric  acid 
and  the  sodium-sulphate  solution  are  in  contact  with  one  another 
in  the  capillary  space  around  the  ungreased  stopcock  which  is 
turned  so  that  connection  is  broken  on  all  sides. 

If  then  the  opening  of  the  mouth  of  the  capillary  tubing  is 
brought  as  close  as  possible  to  the  electrode  whose  potential  it 
is  desired  to  measure,  touching,  for  example,  the  gauze  cathode 
(Fig.  24)  upon  which  a  copper  deposit  is  forming,  and  the  electrode 
is  connected  by  means  of  an  outside  wire  with  the  binding  post 
of  the  auxiliary  electrode,  then  this  combination  forms  a  gal- 
vanic element  consisting  of  mercury  |  mercurous  sulphate  |  electro- 
lyte |  copper. 


42 


QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 


We  have  prepared  in  this  way,  therefore,  the  desired  element 
whose  electromotive  force,  which  is  to  be  measured,  results  from 
the  potential  drop  at  the  cathode  (in  this  case  at  the  coating  of 
copper  upon  the  gauze  electrode)  and  the  potential  drop  at  the 
mercury  of  the  auxiliary  electrode;  the  latter  value  is  known  once 
for  all  time.  Consequently  it  is  necessary  merely  to  measure  the 
electromotive  force  of  this  element  according  to  the  method 
described  on  page  38  and  then  the  potential  of  the  metal  in  question 
against  the  electrolyte  can  be  computed. 


FIG.  11. 

When  an  electrolysis  is  taking  place,  we  have  seen  that  the 
cathode  potential  tends  to  rise  in  the  course  of  the  operation  and 
this  is  disadvantageous  for  the  deposition  of  certain  metals.  It 
is  desirable,  therefore,  to  keep  the  potential  constant,  and  since 
the  potential  of  the  auxiliary  electrode  remains  constant  it  be- 
comes a  question  merely  of  keeping  the  electromotive  force  of  the 
above-described  auxiliary  element  constant  and,  indeed,  at  a  value 
which  has  been  found  favorable  by  previous  investigators. 

This  is  accomplished  by  means  of  the  arrangement  devised  by 
H.  J.  S.  Sand*  to  whom  thanks  are  also  due  for  the  form  of 
auxiliary  electrode  shown  in  Fig.  47  (see  Part  II). 

The  storage  cell  A  is  the  source  of  the  current  in  the  circuit 
ABCD  (Fig.  11),  the  most  important  part  of  which  is  the  sliding 
rheostat  wire  BC.  From  the  latter  starts  the  branch  circuit 
BEKSD,  through  which  a  part  of  the  current  flows  from  the 
storage  cell  and  is  opposed  to  the  electromotive  force  of  the  aux- 

*  The  Rapid  Electrical  Deposition  and  Separation  of  Metals.     Trans- 
actions of  the  Chemical  Society,  91,  380  (1907),  London. 


ELECTROMOTIVE  FORCE  OR  POTENTIAL 


43 


iliary  element  KS.  By  moving  the  sliding  contact  D,  the  potential 
difference  BD  in  the  sliding  rheostat  is  changed  until  it  exactly 
balances  the  electromotive  force  of  KS  as  is  shown  by  the  zero 
reading  of  the  capillary  electrometer  E,  to  be  described  below. 
The  potential  difference  between  the  points  B  and  D  is  then  read 
directly  by  means  of  a  sensitive  voltmeter  which  is  connected  at 
B  and  D.  The  voltage  thus  determined  is  also  the  potential 
difference  of  the  auxiliary  element  KS',  this  value  is  to  be  kept 
constant,  which  is  accomplished  by  regulating  the  current  used  for 
the  analysis  in  the  way  described  under  Bismuth  in  Part  II. 

The  capillary  electrometer  referred  to  is  that  devised  by  Lipp- 
mann*  and  consists,  in  its  most  useful  form,  of  a  small  glass 

flask  (shown  in  Fig.  12,  in  natural  size) 
with  a  capillary  tube  A  fused  to  one  of 
its  sides;  the  capillary  leads  to  the  bot- 
tom of  the  tube  B  which  is  6  mm.  wide. 
The  little  flask  is  half-filled  with  mercury 
and  upon  the  latter  rests  a  saturated 
solution  of  mercurous  sulphate  in  dilute 
sulphuric  acid  (1  vol.  H2S04  :  6  vols. 
H2O) .  The  acid  is  in  contact  with  mer- 
cury at  about  the  middle  of  the  capillary 
tube  and  mercury  is  present  in  the  arm 
B  to  the  height  shown  in  the  figure.  A 
platinum  wire  dips  in  the  mercury  of  the 
arm  B,  and  the  free  end  of  a  second 
platinum  wire,  fused  in  narrow  glass 
tubing  to  prevent  contact  with  the  acid, 
dips  into  the  mercury  at  the  bottom  of 
the  flask.  The  action  of  the  instrument 
as  electrometer  can  be  explained  as  fol- 
lows: Since  mercury  is  a  liquid  that 
does  not  wet  glass,  it  follows  from  the 

laws  of  capillarity  that  the  surface  of  the  mercury  in  the  com- 
municating tubes  A  and  B  will  be  lower  in  the  capillary  tube  than 
in  the  wider  tube.  The  cause  of  this  phenomenon  is  the  surface 
tension  of  the  mercury  which  can  be  imagined  to  act  as  an  elastic 
membrane  surrounding  the  whole  mass  of  mercury.  The  surface 

*  It  is  here  used  as  a  zero  instrument,  i.e.,  not  to  measure  a  current  but  to 
detect  the  absence  of  a  difference  in  potential. 


FIG.  12. 


44  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

tension  strives  to  reduce  the  volume  of  the  mercury  to  a  minimum, 
as  is  evidenced  by  the  curved  surface.  If,  however,  forces  come 
into  play  that  are  opposed  to  this  surface  tension,  then  the  mercury 
level  rises  in  the  capillary  tube  and  the  surface  becomes  natter. 

Without  entering  more  into  the  particulars  of  the  theory  of  the 
instrument,  it  may  be  said  that  differences  in  potential  at  the 
surface  of  the  mercury  change  the  surface  tension,  so  that,  for 
example,  when  a  weak  current  passes  through  the  mercury  of  the 
flask  and  the  sulphuric  acid  to  the  mercury  in  A,  the  level  falls. 
If,  now,  the  current  produced  between  B  and  D  by  the  storage  cell 
in  the  arrangement  shown  in  Fig.  11,  p.  42,  is  to  be  made  equal  to 
that  produced  by  the  element  KS,  it  is  only  necessary  to  move  the 
sliding  contact  D  back  and  forth  until  .the  capillary  electrometer 
shows  the  zero  reading.  To  facilitate  the  accurate  observation 
of  the  mercury  level  in  the  capillary  A,  a  small  microscope  is 
attached  to  the  same  upright  rod  that  holds  the  electrometer. 

The  use  of  the  apparatus  is  further  illustrated  under  the  deter- 
mination of  bismuth. 

Procedure  in  Electro- Analysis.* 
ACTION  OF  THE  CURRENT  UPON  THE  ELECTROLYTE. 

When  it  is  desired  to  accomplish  the  electrolytic  deposition  of  a 
metal  from  a  solution,  the  first  question  that  arises  is:  What  is 
the  most  favorable  composition  of  the  solution  to  be  analyzed? 
Even  in  an  ordinary  gravimetric  analysis  the  nature  of  the  solu- 
tion in  which  the  precipitation  takes  place  is  not  a  matter  of 
indifference.  In  the  case  of  electro-analysis  no  altogether  general 
rules  can  be  given;  in  practice  most  of  the  common  electrolytes 
have  been  studied  with  regard  to  their  qualitative  and  quantitative 
composition  and  in  carrying  out  a  deposition  it  is  not  safe  to  depart 
far  from  the  directions  that  are  given.  To  be  sure,  theory  has 
served  to  clear  up  many  points  although  it  has  not  yet  developed 
enough  to  act  as  the  sole  guide. 

The  preparation  of  the  electrolyte  is  stated,  therefore,  in  every 
case  and  only  a  -few  general  points  will  be  mentioned.  In  the 

*  The  description  and  pictures  of  the  complete  electrical  equipment  of  the 
laboratory  as  given  in  former  editions  of  this  book  is  now  omitted,  for  the 
most  part,  because  the  technique  of  this  branch  of  science  is  progressing 
rapidly  and  forms  of  apparatus  are  rapidly  changing.  There  are  now  a 
number  of  concerns  who  stand  ready  to  supply  all  the  necessary  apparatus. 


PROCEDURE  IN  ELECTRO-ANALYSIS         45 

first  place  it  would  seem  desirable,  when  possible,  to  use  the 
ordinary  salts  of  the  metal  in  the  form  in  which  they  are  present 
in  solution  by  the  preliminary  operations  of  analysis.  The  use  of 
such  solutions  as  the  chlorides  and  sulphates,  however,  is  excep- 
tional, as  will  be  seen  from  the  description  of  the  individual 
methods;  nitrates  are  in  most  cases  wholly  unsuited.  As  regards 
complex  salts,  they  will  be  discussed  a  little  later. 

A  few  examples  will  be  given  here  to  illustrate  the  reactions 
that  take  place  in  electrolytes  through  which  a  current  is  passing, 
whereby,  in  the  sense  of  the  older  theory  of  electrolysis,  "the 
current  decomposes  the  solutions"  (cf.  p.  3). 

SIMPLE  ELECTROLYTES. 

The  passage  of  an  electric  current  through  a  solution  always 
accomplishes  a  chemical  oxidation  at  the  anode  and  a  chemical 
reduction  at  the  cathode.  The  passage  of  96,500  coulombs  of 
electricity  (1  Faraday)  causes  a  gram  atom  of  some  element 
to  gain  one  positive  charge  or  lose  one  negative  charge  at  the 
anode  and  simultaneously  a  gram  atom  of  some  element  loses 
one  positive  charge  or  gains  one  negative  charge  at  the  cathode. 
Meanwhile  the  cations  in  the  solution  are  being  attracted  toward 
the  cathode  and  the  anions  are  migrating  toward  the  anode. 

The  rates  at  which  the  ions  migrate  vary  with  different  ions 
and  usually  the  migration  velocity  of  the  cation  is  different  from 
that  of  the  anion.  During  every  electrolysis,  more  ions  are 
charged  or  discharged  at  each  electrode  in  a  given  interval, 
than  are  brought  to  it  by  the  migration  of  the  ions.  Ions 
in  the  vicinity  of  each  electrode  are  acted  upon  irrespective  of 
whether  they  have  actually  taken  part  in  the  transport  of  elec- 
tricity through  the  solution.  In  many  cases  the  reactions  at 
the  electrodes  are  with  substances  which  are  not  ionized  very 
much  and  which  cannot  take  part  to  any  extent  in  the  move- 
ment of  electricity  through  the  solution. 

This  fact  has  already  been  mentioned  (p.  6)  and  it  is  taken 
up  again  at  this  point  because  it  is  contrary  to  views  which  once 
prevailed. 

The  reduction  that  takes  place  at  the  cathode  or  the  oxidation 
that  takes  place  at  the  anode  is  always  the  easiest  oxidation  or 
reduction  which  it  is  possible  to  accomplish  under  the  prevailing 
conditions.  The  conductivity  of  the  solution  depends  upon  the 


46  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

presence  of  ions  in  the  solution,  but  many  substances  are  capable 
of  oxidation  and  reduction  which  are  not  ionized  to  any  extent. 
The  table  on  page  26  shows  the  oxidation  potential  of  various 
metals  in  contact  with  solutions  of  their  ions  molal.  To  reduce 
these  metals  from  the  ionic  condition  to  that  of  the  free  metal 
it  is  necessary  to  overcome  the  oxidation  potential  of  the  metal. 
In  other  words,  the  decomposition  potential  is  reached  as  soon 
as  the  oxidation  potential  is  overcome.  The  metals  at  the  bot- 
tom of  the  series,  therefore,  are  the  ones  which  it  is  easiest  to 
deposit  upon  the  cathode.  The  Nernst  formula,  page  27,  shows 
that  this  oxidation  potential  increases  as  the  solution  is  more 
dilute;  it  follows,  therefore,  that  more  electromotive  force  is 
required  to  discharge  ions  from  a  dilute  solution  than  from  a 
concentrated  one. 

If  a  solution  of  copper  sulphate  is  subjected  to  electrolysis 
the  conductivity  of  the  solution  is  due  to  the  presence  of  cupric 
and  sulphate  ions;  the  former  migrate  toward  the  catnode  and 
the  latter  toward  the  anode.  In  a  0.1-normal  solution  the 
cupric  ions  move  about  0.6  as  fast  as  the  sulphate  ions.  The 
passage  of  96,500  coulombs  of  electricity  from  pole  to  pole  will 
be  accompanied  by  a  movement  of  the  ions  in  proportion  to  their 
rates  of  migration;  96,500  X  0.625  coulombs  will  be  carried 
by  the  anions  and  the  balance,  96,500  X  0.375  coulombs  will 
be  carried  by  the  cations.  At  the  cathode  a  gram  equivalent  of 

o  4-     \jyi~ 

copper,  —  '—  —  '  =  31.5  grams,  will  be  deposited  by  this  96,500 

coulombs  of  electricity.  Adopting  the  convention  of  repre- 
senting a  unit  charge  of  positive  or  negative  electricity  by  the 
symbols  0  and  ©,  the  reduction  at  the  cathode  may  be  expressed 
as  follows: 

20  =  Cu. 


In  the  copper  sulphate  solution  the  only  other  conceivable  re- 
ductions would  be  that  of  hydrogen  from  water  or  sulphur  from 
the  sulphate.  Both  of  these  last  two  reductions  are  harder  to 
accomplish  than  that  of  cupric  ions  to  the  metallic  state.  The 
table  on  page  26  shows  that  copper  is  below  hydrogen  in  the 
potential  series;  hydrogen,  moreover,  has  a  much  higher  oxidation 
potential  against  the  low  concentration  of  hydrogen  cations  in  pure 
water  than  against  a  normal  solution  of  hydrogen  ions.  If  the 


PROCEDURE  IN  ELECTRO-ANALYSIS  47 

copper  sulphate  solution  contained  free  sulphuric  acid,  however, 
the  time  might  come  when  it  would  be  easier  to  discharge  hydro- 
gen ions  from  the  acid  than  to  discharge  the  cupric  ions  from  the 
dilute  solution. 

At  the  anode  the  easiest  oxidation  depends  somewhat  upon  the 
nature  of  the  electrode.  If  a  copper  anode  is  used  in  the  electrolysis 
of  a  copper  sulphate  solution,  the  easiest  oxidation  will  be  that 
of  copper  from  the  metallic  to  ionic  condition. 

Cu  +  2  0  =  Cu  +  +. 

In  this  case,  the  same  quantity  of  copper  dissolves  at  the  anode 
as  is  deposited  at  the  cathode  and  the  total  concentration  of 
the  solution  in  cupric  ions  remains  unchanged.  If,  on  the  other 
hand,  the  anode  is  platinum,  this  metal  does  not  dissolve  easily 
as  the  oxidation  potential  is  very  low.  The  only  other  possi- 
bilities are,  first,  the  discharge  of  the  sulphate  ions,  second,  an 
oxidation  of  the  sulphur,  third,  an  oxidation  of  oxygen.  The 
elements  hydrogen  and  copper  are  already  in  their  highest  state 
of  oxidation.  As  regards  the  first  possibility,  there  is  no  good 
evidence  that  free  S04  can  exist  by  itself.  It  has  often  been 
assumed  that  864  =  anion  can  be  discharged  and  that  it  imme- 
diately reacts  with  water  but  this  assumption  does  not  seem 
reasonable  when  all  the  facts  are  considered.  The  second  pos- 
sibility is  that  of  the  formation  of  persulphate  ions  and  under 
certain  conditions  this  does  take  place: 

2S04=  +  20  =  S208  = 

Under  ordinary  conditions,  however,  this  is  not  the  easiest  oxida- 
tion and  there  is  no  appreciable  quantity  of  persulphate  anions 
formed.  In  the  electrolysis  of  a  dilute  aqueous  solution  of  copper 
sulphate  between  platinum  electrodes  the  easiest  oxidation  is 
that  of  oxygen  from  the  negative  condition  to  that  of  neutral 
oxygen  gas: 


The  behavior  of  sodium  chloride  solution  upon  electrolysis 
has  already  been  mentioned  (page  5).  The  table  of  oxidation 
potentials  (page  26)  shows  that  sodium  ions  are  much  harder 
to  discharge  than  hydrogen  ions.  The  difference  in  oxidation 
potentials  is  so  great  that,  in  accordance  with  the  Nernst  formula 


48  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

(page  27),  it  is  easier  to  discharge  hydrogen  from  water  than 
sodium  ions  from  a  normal  solution  of  sodium  chloride.  In  fact, 
metallic  sodium  decomposes  water  because  its  oxidation  poten- 
tial is  greater  than  that  of  hydrogen  toward  water.  In  the 
electrolysis  of  sodium  chloride  solution,  therefore,  the  current  is 
carried  from  pole  to  pole  by  the  sodium  and  chloride  ions.  At 
the  cathode,  unless  it  is  composed  of  mercury,  the  easiest  reduction 
is  that  of  hydrogen: 

2H2O  +  20  =  20H-  +  H2. 

If,  however,  a  mercury  cathode  is  used  it  is  possible  to  obtain 
sodium  amalgam.  This  simply  means  that  it  is  easier  to  reduce 
sodium  from  the  ionic  condition  in  water  to  that  of  sodium  dis- 
solved in  mercury  than  it  is  to  reduce  sodium  from  the  ionic 
condition  to  that  of  the  free  element.  It  does  not  prove,  as  has 
been  argued  falsely,  that  sodium  is  always  set  free  momentarily 
and  then  decomposes  water. 

At  the  anode,  chlorine  is  set  free  during  the  electrolysis  of 
sodium  chloride  provided  the  anode  is  not  attacked. 

2C1~  +  20  =  C12. 

As  the  concentration  of  the  Cl~  anions  in  the  solution  decreases, 
the  decomposition  voltage  increases  and  eventually  it  becomes 
easier  to  discharge  oxygen  from  water  than  to  discharge  chlorine 
from  the  dilute  solution.  Moreover,  if  the  conditions  are  fa- 
vorable, the  oxidation  at  the  anode  may  change  the  chlorine  into 
hypochlorite  anions  or  even  chlorate  anions. 

OP  +  H20  +  2  0  =  CIO"  +  2H+, 
CIO-  +  3H20  +  60  =  C103"  +  6  H+. 

Copper  is  sometimes  deposited  electrolytically  from  a  solu- 
tion containing  free  nitric  acid.  Nitric  acid  itself  is  susceptible 
of  cathodic  reduction  and,  indeed,  the  reduction  of  the  nitrogen 
may  go  from  the  quinquivalent  positive  condition  to  that  of 
trivalent  negative  nitrogen  in  ammonia  or  ammonium  salt  (in 
the  latter  the  nitrogen  has  four  negative  charges  and  one  positive 
charge) : 

N03-  +    9H+  +  80  =  NH3  +  3H20. 
N03-  +  10H++  80  =  NH4+  +  3H20. 


PROCEDURE  IN  ELECTRO-ANALYSIS  49 

If  the  conditions  are  such  that  any  considerable  quantity  of 
nitrous  acid  is  present  in  the  solution  at  any  time,  this  compound 
is  so  easily  reduced  that  it  will  not  only  interfere  with  further 
deposition  of  the  copper  but  will  cause  the  oxidation  and  solution 
of  copper  which  has  already  been  deposited.  It  is  because  of 
this  possibility  of  forming  nitrous  acid  at  the  cathode  that  nitric 
acid  solutions  are,  in  general,  avoided  for  electrolytic  operations. 
Nitric  acid  at  the  anode  is  stable  and  the  easiest  oxidation 
in  the  electrolysis  of  nitric  acid  solutions  is  that  of  the  oxygen 
from  water;  it  is  evolved  as  gas  as  in  the  electrolysis  of  sul- 
phuric acid  solutions. 

It  is  often  true  that  a  reduction  or  oxidation  once  started 
will  go  beyond  the  primary  stage.  After  two  positive  charges 
have  been  neutralized  on  the  nitrogen  atom  in  the  nitrate 
anion,  it  is  easier  to  neutralize  the  remaining  positive  charges 
than  it  was  to  take  away  the  first  two.  An  analogous  condition 
is  in  the  discharge  of  the  cupric  ion.  The  decomposition  of  po- 
tential of  cuprous  ions  is  so  low  that  most  cuprous  compounds 
cannot  exist  in  aqueous  solution  except  in  low  concentrations. 
In  other  words,  the  stable  cuprous  salts  are  not  very  soluble  in 
water.  In  the  electrolysis  of  solutions  containing  cupric  ions, 
therefore,  cathodic  reduction  will  cause  deposition  of  metallic 
copper  because  it  is  easier  to  neutralize  two  positive  charges  on 
one  cupric  cation  than  to  neutralize  one  positive  charge  on  two 
cupric  ions.  In  the  electrolysis  of  a  cupric  chloride  solution,  how- 
ever, this  is  not  the  case.  As  the  cupric  ion  loses  one  charge 
it  enters  into  equilibrium  with  the  chlorine  anions  and  insoluble 
cuprous  chloride  is  formed  and  the  concentration  of  the  cuprous 
ions  is  so  low  that  it  is  easier  to  reduce  fresh  cupric  ions  than  to 
deposit  metallic  copper. 

The  presence  of  organic  substances  in  solutions  undergoing 
electrolysis  often  has  an  effect  upon  the  products  obtained  at 
the  anode  and  at  the  cathode.  Some  of  these  substances,  such 
as  the  salts  of  organic  acids,  are  electrolytes  and  take  part  in 
the  conduction  of  the  current;  they  are  also  subject  to  oxidation 
and  reduction  at  the  electrodes.  At  the  cathode  it  is  possible, 
for  example,  to  reduce  nitrobenzene,  CeHsNC^,  to 
C6H5NO,  C6H5NHOH,  C6H5N  —  NC6H5,  C5H5N— NC6H5, 

\0/ 
C6H5NH  •  NHC6H5,  H2NC6H5  •  C6H5NH2  and  finally  to  C6H5NH2. 


50  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

The  complete  reduction  of  nitrobenzene  to  aniline  may  be  ex 
pressed  as  follows: 

C6H5NO2  +  6H+  +  60  =  C6H5NH2  +  2H2O. 

Electrolysis  of  a  solution  containing  acetate  ions  results  in  the 
formation  of  ethane  and  carbon  dioxide  at  the  anode: 

2C2H302-  +  20=  C2H6  +  2C02. 

Similarly,  succinate  ions  may  be  oxidized  to  ethylene  and  carbon 
dioxide  : 

C2H4(C02)2=  +20=  C2H4  +  2C02. 

Lactate  ions  are  changed  to  acetaldehyde  and  carbon  dioxide: 

C3H503~  +  OH~  +  20  =  CH3CHO  +  C02  +  H20. 
Oxalate  ions  are  changed  to  carbon  dioxide: 
C204=  +  2  ©  =  2C02. 

In  the  decomposition  of  an  alkali  oxalate,  hydrogen  is  set  free 
at  the  cathode  from  water 

H20  +  2  0  ->  H2  +  20H-, 

and  some  of  the  C02  evolved  at  the  anode  may  react  with  the 
hydroxyl, 

-  +  C02->2HC03-. 


COMPLEX  ELECTROLYTES 

It  is  sometimes  desirable  to  electrolyze  a  solution  containing 
a  metal  in  the  state  of  complex  ions.  Thus  copper  may  be  present 
as  copper  ammonia  ions,  Cu(NHs)4++.  Such  a  solution  requires 
more  electromotive  force  to  reduce  the  copper  to  the  metallic 
condition  because  the  discharge  potential  of  the  copper  is  greater 
in  proportion  as  the  solution  contains  fewer  cupric  ions.  On 
the  other  hand,  it  is  much  harder  to  discharge  -hydrogen  from 
the  ammoniacal  solution  than  from  an  acid  solution  so  that  there 
is  less  danger  of  the  nature  of  the  deposit  being  influenced  by  the 
simultaneous  deposition  of  hydrogen  with  the  copper.  In  the 
case  of  nickel,  it  is  impossible  to  deposit  this  element  from  an 
acid  solution  as  it  is  easier  to  discharge  hydrogen  ions  but  from 


PROCEDURE  IN  ELECTRO-ANALYSIS  51 

an  ammoniacal  solution  containing  nickel-ammonia  ions  all  of 
the  nickel  can  be  deposited  on  the  cathode. 

The  situation  is  somewhat  more  complicated  in  the  electrolysis  of 
a  slightly  alkaline  solution  of  potassium  cuprocyanide,  K3Cu(CN)4. 
This  salt  in  aqueous  solution  ionizes  as  follows: 

K3Cu(CN)4  <=*  3K+  +  Cu(CN)4s. 

The  Cu(CN)4s  ions  are  also  in  equilibrium  with  Cu+  and  CN~ 
ions,  but  whereas  the  primary  ionization  takes  place  to  a  very 
considerable  extent,  it  has  been  estimated  that  the  ratio  of  the 
concentration  of  the  complex  ion,  Cu(CN)4T,  to  simple  cuprous 
ion,  Cu+,  in  a  normal  solution  of  potassium  cyanide  is  as  1026  :  1. 
The  discharge  potential  of  cuprous  ions,  however,  is  much  lower 
than  that  of  cupric  ions  of  equivalent  concentration. 

As  far  as  the  conduction  of  the  electric  current  is  concerned, 
the  Cu(CN)4^  anions  migrate  toward  the  anode.  They  are, 
however,  not  discharged  there  if  the  solution  contains  simple 
cyanide  ions,  because  the  easiest  oxidation  is  as  follows: 

2CN~  +  20  =  (CN)2. 

At  the  cathode,  on  the  other  hand,  it  is  easier  to  discharge  cuprous 
ions  of  very  low  concentration  than  potassium  ions  of  high  con- 
centration so  that  the  reaction  at  the  cathode  may  be  expressed 
as  follows : 

Cu(CN)4"  +  0  =  Cu  +  4CN~ 

It  is  easy  to  understand  that  a  higher  potential  and  higher 
current  strength  will  be  necessary  to  deposit  the  copper  from 
such  a  complex  ion  than  from  that  of  a  simple  copper  salt.  If, 
on  the  other  hand,  it  is  desired  to  separate  copper  from  cadmium 
by  electrolysis,  it  is  possible  to  change  the  order  of  deposition. 
In  an  acid  solution  the  copper  can  be  deposited  quantitatively 
and  no  cadmium  ions  will  be  discharged  as  long  as  the  solution 
remains  acid.  In  a  potassium  cyanide  solution  cadmium  forms 
complex  Cd(CN)4=  anions,  but  the  ratio  of  the  concentration 
of  the  complex  ion  to  that  of  simple  cadmium  ions  in  a  normal 
potassium  cyanide  solution  is  about  1017  :  1  and  it  is  easier  to 
deposit  cadmium  from  such  a  solution  than  copper.  By  stirring 
the  solution  it  is  possible  to  keep  some  of  the  anions  in  the  vicinity 
of  the  cathode  even  although  the  current  tends  to  carry  them 
toward  the  anode. 


52  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Potassium  cyanide  solution  is  an  excellent  solvent  for  silver 
salts  and  such  solutions  are  much  used  for  silver  plating.  The 
silver  exists  in  such  solutions  in  the  form  of  Ag(CN)2~  anions. 
If  a  potassium  cyanide  solution  of  a  silver  salt  is  electrolyzed 
with  a  silver  anode,  the  easiest  oxidation  at  this  electrode  will 
be  the  solution  of  the  silver: 

Ag  +  2CN-  +  ®  =  Ag(CN)2~ 

By  stirring,  these  ions  can  be  carried  to  the  vicinity  of  the  cathode 
and  then,  although  only  very  few  simple  silver  cations  are  present, 
it  is  easier  to  discharge  the  silver  than  to  accomplish  the  reduc- 
tion of  potassium  or  of  hydrogen  from  an  alkaline  solution 

Ag(CN)2-  +  0  =  Ag  +  2CN-. 

The  character  of  the  silver  deposit  is  more  satisfactory  and  less 
granular  than  when  formed  by  the  electrolysis  of  an  acid  solu- 
tion of  a  simple  salt. 


Character  of  the  Metal  Deposit  and  Duration  of  the 
Electrolysis. 

Two  important  points  to  be  considered  in  electro-analysis  are 
the  nature  of  the  deposit  and  the  duration  of  the  electrolysis;  these 
two  factors  are  closely  related  to  one  another. 

As  regards  the  nature  of  the  deposit,  it  is  absolutely  necessary 
that  it  shall  adhere  firmly  to  the  cathode  in  order  that  the  solution 
with  which  it  is  wet  may  be  rinsed  off  without  loss.  The  most 
favorable  form  of  the  deposit  in  this  respect  is  the  finely  crystal- 
line one,  with  metallic  luster  if  possible.  Dull  deposits  are  less 
dense,  and,  on  account  of  their  pulverulent  nature,  more  likely 
to  become  spongy.  If  the  deposit  is  distinctly  spongy,  then  it 
adheres  loosely  to  the  cathode  and  this  is  why  spongy  deposits 
should  be  discarded.  The  principal  cause  of  the  sponginess  lies 
in  the  too  rapid  deposition  of  the  metal.  It  is  conceivable  that 
under  these  conditions  the  precipitate  does  not  have  time  to  as- 
sume a  finely  crystalline  form.  Formerly,  attention  was  directed 
chiefly  toward  the  current  density,  i.e.,  to  the  number  of  am- 
peres per  square  decimeter  of  cathode  surface.  Mention  of  the 
current  density  was,  and  is  also  to-day,  an  important  factor 
for  certain  depositions.  The  quantity  of  metal  deposited  is, 


DURATION  OF  THE  ELECTROLYSIS  53 

according  to  Faraday's  law  (p.  9),  directly  proportional  to  the 
current  strength;  thus  a  current  of  two  amperes  will  deposit 
twice  as  much  metal  in  a  unit  of  time  as  will  be  deposited  by  a 
current  of  one  ampere  during  the  same  time.  This  is  one  reason 
why  too  strong  current  densities  fa,vor  the  formation  of  spongy 
deposits.  Currents  of  high  intensity  have  the  further  disadvan- 
tage of  favoring  the  evolution  of  hydrogen  at  the  cathode,  which 
also  hinders  the  uniform  deposition  of  the  metal  (cf.  p.  22). 
Finally,  it  may  happen  under  these  conditions  that  metal  hydrides 
are  formed  at  the  cathode,  and  these  hydrides  are  subsequently 
decomposed  with  evolution  of  hydrogen  leaving  the  metal  behind 
in  a  less  compact  condition.  It  must  be  remembered,  moreover, 
that  the  deposition  of  the  metal  can  take  place  strictly  in  accord- 
ance with  Faraday's  law  only  during  the  first  few  moments  of 
the  analysis,  for  as  soon  as  some  of  the  metal  has  deposited,  the 
composition  of  the  solution  becomes  changed.  The  current  then 
acts  upon  this  solution  somewhat  differently  than  it  did  upon  the 
solution  in  its  original  composition;  this  is  evidenced  by  the 
evolution  of  hydrogen  which  increases  in  amount  as  the  quantity 
of  metal  in  the  solution  becomes  less;  the  last  portions  of  the 
metal,  therefore,  require  a  relatively  longer  time  for  deposition 
than  the  first  portions.  It  is  not  infrequent,  for  this  reason,  to 
have  the  analysis  prolonged  two,  four,  or  even  six  hours,  according 
to  the  nature  of  the  metal  and  the  quantity  to  be  deposited. 

Now  the  shortening  of  the  time  required  to  effect  the  quantita- 
tive deposition  of  a  metal  is  a  factor  of  great  importance  which  has 
received  the  attention  of  investigators  for  a  long  time.  The  result 
of  the  numerous  investigations  in  this  field,  concerning  which  a 
historical  summary  will  shortly  be  given,  has  placed  us  to-day  in 
a  position  of  being  able  to  complete  an  electrolysis  in  about  one 
fifteenth  of  the  time  formerly  employed.  This  result  has  been 
accomplished  by  the  use  of  rapidly  rotating  electrodes,  or,  what 
amounts  to  the  same  thing,  of  a  rapidly  moving  electrolyte;  the 
important  point  in  all  cases  is  the  movement  of  the  electrolyte. 

Shape  of  the  Electrodes. 

A  great  many  differently  shaped  electrodes  have  been  proposed 
from  time  to  time,  but  only  a  few  forms  have  met  with  favor  in 
practice.  Here,  a  few  electrodes  will  be  described  first  which  are 


54  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

employed  for  ordinary  electrolytic  methods  when  the  work  is 
carried  out  without  stationary  electrolytes. 


FIG.  13.  FIG.  14.      < 

Among  the  oldest  models  are  those  used  at  the  Mansfeld  smelt- 
ing works,  chiefly  for  the  determination  of  copper.  The  cathodes 
are  made  of  platinum  foil  bent  into  the  shape  of  a  cone  or  cylinder, 
with  a  few  slits  in  the  foil  to  facilitate  the  circulation  of  the  liquid. 
Strong  platinum  wires  are  riveted  or  soldered  to  the  foil;  plati- 
num can  be  united  to  platinum  without  the  use  of  a  foreign  solder 
(Figs.  13  and  14).  The  corresponding  anodes,  made  of  stout 
platinum  wire,  are  shown  in  Figs.  15  and  16.  Figures  17  and  18 
show  how  these  electrode  pairs,  attached  to  electrode  stands, 
are  arranged  in  an  electrolyte  contained  in  a  beaker.  Another 
method  is  to  use  a  single  stand,  as  shown  in  Fig.  19.  This  arrange- 
ment is  a  very  practical  one  if  a  metal  is  to  be  deposited  from  a 
slightly  acid  solution;  when  the  electrolysis  is  complete,  the  stand 
is  quickly  raised  so  that  the  attached  electrodes  are  removed  from 
the  liquid  and  then  they  are  quickly  immersed,  without  breaking 
the  circuit,  first  in  a  beaker  filled  with  water,  and  next  in  one  con- 
taining alcohol;  it  is  then  only  necessary  to  dry  the  electrodes  for  a 
short  time  in  an  air-bath  before  weighing.  When  a  single  stand 
is  used,  the  rod  G  (Fig.  19)  must  be  made  of  glass. 

Besides  these  types  of  electrodes,  the  use  of  platinum  dishes,  as 
recommended  by  the  author,  has  met  with  much  favor.  Fig.  20 
shows  such  a  platinum  dish  in  half  its  natural  size;  it  weighs  about 
35  grams,  has  a  diameter  of  about  9  cm.,  is  4.2  cm.  deep  in  the 


SHAPE  OF  THE  ELECTRODES 


55 


center  and  holds  about  150  cc.     With  150  cc.  in  the  dish  the  wet 
inner  surface  amounts  to  about  100  sq.  cm.,  and  with  180  cc.  to 


FIG.  15. 


FIG.  16. 


FIG.  17. 


56 


QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 


about  150  sq.  cm.  As  it  has  been  shown  that  most  metals  adhere 
better  to  a  slightly  rough  surface  than  to  a  polished  one,  the  in- 
side of  the  dish  is  preferably  roughened  by  means  of  the  sand  blast. 
In  certain  determinations,  as  in  the  deposition  of  lead  peroxide, 
the  deposit  will  adhere  firmly  only  to  such  a  dull  surface. 


FIG.  18. 

It  is  advisable,  under  all  circumstances,  to  reserve  for  this  pur- 
pose only  the  dishes  used  in  electro-analysis  and  to  take  care  that 
they  are  not  dented  or  bent  by  careless 
handling.  Dishes  made  of  an  alloy  of 
platinum  with  10  per  cent  iridium  are 
not  so  sensitive  in  this  respect  as  are 
those  of  the  softer,  pure  platinum. 

As  anode  (positive  electrode)  the 
author  uses  a  disk,  4.5  cm.  in  diameter, 
made  of  fairly  thick  platinum  foil  which 
is  riveted  or  autogenously  soldered  to 
a  quite  stout  platinum  wire  (Fig.  21). 
For  the  reason  already  mentioned  in 
the  description  of  conical  and  cylindri- 
cal electrodes,  it  is  well  to  provide  the 
platinum  disk  with  a  number  of  slits. 
On  account  of  the  horizontal  position 
of  the  disk  anode  in  the  electrolyte,  if 
these  slits  are  not  provided  the  bubbles 
of  gas  that  collect  beneath  the  disk  will  diminish  the  contact  sur- 
face between  the  solution  and  electrode  and  thereby  increase  the 


FIG.  19. 


SHAPE  OF  THE  ELECTRODES 


57 


FIG.  20. 


resistance  of  the  cell.    When,  eventually,  these  tiny  bubbles  of 
gas  unite  to  form  one  large  bubble,  this  may  escape  from  under  the 

platinum  so  quickly  that  there 
is  danger  of  losing  some  of  the 
liquid  by  spattering. 

Besides  the  disk-shaped  anode 
shown  in  Fig.  21,  the  author  also 
uses  an  anode  of  the  form  shown 
in  Fig.  22,  which  consists  of  a 
perforated  platinum  dish  about 
50  mm.  in  diameter  and  20  mm.  deep.  This  anode  has  been  used 
by  Julia  Langness  as  a  rotating  anode.  The  use  of  a  platinum 
dish  as  cathode  has  the  advantage  that  in  working  with  moving 
electrolytes  the  anode  may  be  chosen  of  almost  any  form  accord- 
ing to  the  special  effect  that  it  is  desired  to  accomplish. 

To  hold  the  electrodes  in  position,  two  special  stands  have  been 
designed  by  other  investigators  (Figs.  17  and  18).  The  author 
has  combined  these  on  a  single  stand  which  has  proved  satisfac- 
tory. The  ring  which  serves  to  support  the  dish  (Fig.  23)  is 


FIG.  23. 


provided  with  three  short  platinum  contacts,  and,  like  the  arm 
that  holds  the  anode,  is  fastened  to  a  vertical  glass  rod  G;  n  is 
connected  with  the  negative  and  p  with  the  positive  pole  of  the 
source  of  current. 


58  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

It  must  be  admitted  that  the  dish  form  of  cathode  has  certain 
disadvantages.  In  the  first  place,  the  circulation  of  the  liquid  is 
likely  to  be  unsatisfactory.  At  several  places  in  this  book  it  has 
been  pointed  out  that  the  metal  ions  should  be  supplied  to  the 
cathode  as  fast  as  possible,  not  only  in  the  interest  of  shortening 
the  time  of  analysis  but  also  for  obtaining  a  satisfactory  deposit. 
If  the  supply  of  metal  ions  were  maintained  solely  by  an  equali- 
zation of  the  different  densities  which  the  liquid  assumes  during 
electrolysis,  then  the  dish  form  of  electrode  would  be  advantageous 
because  the  upper  layers  of  liquid  become  richer  in  metal  and 
consequently  denser  than  the  lower  layers,  and  as  a  result  the 
metal  ions  tend  to  sink  of  their  own  accord  to  the  lower  levels 
where  they  are  needed.  A  much  more  energetic  mixing  of  the 
solution,  however,  is  brought  about  by  the  bubbles  of  gas  that  are 
evolved  during  the  electrolysis.  If  it  be  assumed  that  during  a 
well-conducted  electrolysis  there  is  no  evolution  of  hydrogen  at 
the  cathode,  then  it  is  only  the  oxygen  bubbles  evolved  at  the 
anode  that  can  serve  to  stir  the  solution.  Herein  lies  the  fault 
of  the  dish  as  electrode,  for  with  it  this  stirring  takes  place  only 
in  the  upper  portions  of  the  electrolyte.  Some  advantage  is 
gained,  however,  by  warming  the  solution.  If,  however,  the 
electrolyte  is  constantly  stirred  during  the  electrolysis,  this  objec- 
tion to  the  use  of  a  dish  electrode  disappears. 

As  a  result  of  experience,  it  has  been  found  that  certain  pre- 
cipitates, which  are  often  formed  during  the  preparation  of  the 
electrolyte,  do  not  influence  the  character  of  the  metal  deposit 
obtained,  so  that  it  is  unnecessary  to  waste  time  by  filtering, 
washing  and  evaporating.  In  such  cases,  the  use  of  a  platinum 
dish  as  cathode  would  have  the  disadvantage  of  having  the  precipi- 
tate rest  upon  the  metal  deposit,  which  could  easily  give  rise  to 
contamination  of  the  latter.  Energetic  mechanical  stirring  would 
tend  to  obviate  this  difficulty.* 

At  the  present  high  price  of  platinum,  a  final  objection  to  the 
use  of  dish  electrodes  lies  in  the  fact  that  only  about  one  third  of 
the  total  surface  of  the  platinum  is  utilized  in  an  electrolysis. 
Moreover,  a  heavier  weight  of  platinum  is  not  altogether  desirable. 

In  spite  of  these  various  objections  that  have  been  raised,  a 

*  The  results  of  recent  experiments  with  respect  to  the  determination  of 
metals  in  the  presence  of  suspensions  have  often  conflicted  with  older  observa- 
tions, as  will  be  discussed  in  the  case  of  certain  metal  separations. 


SHAPE  OF  THE  ELECTRODES  59 

number  of  authorities,  such  as  Hollard  and  Bertiaux,  Riban, 
Exner,  E.  F.  Smith,  R.  O.  Smith,  Langness,  Ingham  and  others, 
have  obtained  excellent  results  with  dishes  as  cathodes  and 
various  types  of  anodes. 

With  respect  to  the  nature  of  the  inner  surface  of  the  dishes,  the 
author  at  first  used  highly  polished  dishes.  After  it  was  dis- 
covered that  these  polished  surfaces  were  not  suitable  for  holding 
large  deposits  of  certain  metals  (e.g.,  antimony)  and  still  less  so 
for  holding  peroxides  (of  manganese  or  lead) ,  the  author  became 
accustomed  to  the  use  of  dishes  that  had  been  dulled  by  sand 
blasting.  More  recently,  however,  careful  experiments  carried 
out  in  his  laboratory,  especially  in  the  determination  of  antimony, 
have  shown  that  if  the  surface  is  roughened  too  much  there  is 
danger  of  some  of  the  salts  contained  in  the  electrolyte  being  in- 
cluded in  the  deposit.  For  this  reason  the  author  now  recommends 
that  the  inner  surface  of  the  dish  be  only  slightly  dulled,  as  can  be 
accomplished  by  warming  slightly  with  dilute  aqua  regia.  (See 
Antimony.) 

This  last  difficulty  is  much  more  serious  in  the  case  of  gauze 
electrodes  which,  according  to  their  nature,  may  seriously  in- 
fluence the  accuracy  of  the  results  by  giving  rise  to  foreign  inclu- 
sions. 

Since  1898,  wire  gauze  electrodes,  especially  cathodes,  have  been 
used.  H.  Paweck*  overcame  the  difficulties  encountered  in  the 
electrolytic  estimation  of  zinc,  by  the  use  of  a  disk-shaped  cathode 
made  of  brass  gauze  previously  amalgamated,  and  he  was  also  able 
by  means  of  such  cathodes  to  obtain  satisfactory  zinc  deposits 
from  an  alkaline  tartrate  solution  or  from  a  slightly  acid  sulphate 
solution.  The  gauze  proved  suitable  for  a  good  amalgamation. 

Cl.  Winkler  used  platinum  gauze  and  made  the  electrode 
cylindrical  in  form  (Fig.  24);  as  anode  he  used  a  stout  platinum 
wire  wound  into  a  spiral  (Fig.  25).  The  above-mentioned  error 
occasioned  by  foreign  inclusions  is  partly  but  not  entirely  avoided 
if  the  meshes  of  the  gauze  are  not  too  fine,  and  if  the  edges  of  the 
cylinder,  instead  of  being  bent  over,  as  was  formerly  customary, 
are  soldered  to  a  round  platinum  wire. 

The  advantage  that  wire  gauze  electrodes  possess  over  those 
made  from  platinum  foil  bent  into  cylinders  or  cones  consists  in 
a  uniform  distribution  of  the  current  upon  the  inside  and  outside. 
*  Z.  Elektrochem.  6,  221  (1896). 


60 


QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 


To  still  further  aid  in  the  uniform  distribution  of  the  current, 
Hollard  uses  an  anode  of  the  form  shown  in  Fig.  23,  a  part  of 
which  is  inside  and  a  part  outside  the  cathode. 


FIGS.  24  and  25. 


FIG.  26. 


FIGS.  27  and  27a. 


For  the  same  purpose  of  surrounding  the  cathode  by  the  anode 
as  much  as  possible,  F.  M.  Perkin  uses  a  gauze  cathode  shaped  like 
a  flag  (Fig.  27)  and  a  fork-shaped  anode  of  platinum  wire  bent  as 
shown  in  Fig.  27a;  the  cathode  can  thus  be  inserted  between  the 
windings  of  the  anode.  (Oettel  had  previously  recommended  a 
fork-shaped  anode.)  The  wire  gauze  of  the  cathode  is  soldered  to 
a  frame  of  platinum  wire.  The  loop  in  the  upper  part  of  the 
cathode  is  for  the  purpose  of  hanging  it  to  the  balance  arm. 

The  electrodes  used  for  rapid  electro-analysis  will  be  described 
below. 

Electro-Analysis  with  Moving  Electrolytes.      (Rapid  Electro- 
Analysis.) 

In  analyses  made  with  the  electrodes  already  described,  it  was 
formerly  customary  to  allow  the  electrolytes  to  stand  quietly  until 
the  deposition  was  complete.  The  movement  that  naturally 
takes  place  within  such  a  solution  is  caused  by  the  ascending  gas 
bubbles  and  by  diffusion,  the  latter  being  caused  by  the  fact  that 
the  solution  in  the  neighborhood  of  the  cathode  becomes  less 
concentrated  and  specifically  lighter  than  the  portions  of  liquid 
farther  away.  Some  years  ago  the  author  pointed  out  the  favor- 


ELECTRO-ANALYSIS  WITH   MOVING  ELECTROLYTES        61 


able  effect  due  to  heating  the  electrolyte.  The  increased  rate  of 
deposition  from  a  hot  solution  is  due,  however,  to  the  increased 
conductivity  caused  by  heating  the  solution,  or  to  a  decreased 
resistance,  and  to  an  accelerated  rate  of  diffusion.  Subsequently, 
experiments  were  undertaken  with  rapidly  moving  electrolytes 
and  surprising  results  were  obtained  as  regards  the  shortening 
of  the  time  required  for  complete  deposition.  After  the  experi- 
ments performed  in  the  author's  laboratory  and  elsewhere  had 
removed  all  doubt  concerning  the  value  of  the  new  method  of 
working,  the  authorities  at  Aachen  consented  to  provide  means  for 
fitting  up  the  first  large  laboratory  with  the  necessary  apparatus 
for  carrying  out  rapid  electrolyses  (see  p.  68). 

Although  much  remains  to  be  explained  in  the  theory  of  rapid 
electrolysis,  still  the  experiments  made  in  the  past  to  explain  the 
observed  facts  in  the  light  of  known  theories  are  worthy  of  careful 
consideration.  This  is  not  the  place  to  go  into  such  matters  in 
detail  and  we  shall  limit  the  discussion  to  the  principal  results 
that  have  been  obtained  by  investigations  in  this  direction  and 
shall  refer,  in  the  discussion  of  the  individual  methods,  to  the 
original  papers  by  appropriate  footnotes.  R.  Amberg,*  who,  in 
1903,  carried  out  methodical  determinations  of  palladium  by 
rapid  electrolytic  methods,  gives  in  his  thesis  the  following  table 
in  which  the  second  column  gives  the  weight  of  deposited  platinum 
in  grams,  the  column  headed  Zw  gives  the  time  required  in  each 
case  and  the  last  column  gives  the  number  of  revolutions  of  the 
stirrer  per  minute. 

In  the  column  headed  Zf  is  found  the  theoretical  time,  in  hours, 
required  for  the  deposition  if  it  took  place  with  the  best  possible 
utilization  of  the  current  in  accordance  with  Faraday's  law  (see 
p.  9).  Column  Zw  —  Zf  gives  the  difference  between  the  actual 
time  required,  as  given  in  column  Zw,  and  the  computed  time  in 
column  Z. 


No. 

Grams  Pd 
deposited. 

%w 

zf. 

zw  -  zf. 

Revolutions  per 
minute. 

1 

0.77 

5.50 

1.55 

3.95 

500 

2 

0.6 

4.45 

1.20 

3.25 

620 

3 

0.95 

4.5 

1.91 

2.49 

800 

4 

2.3 

6.0 

4.62 

1.38 

1000 

Z.  Electrochem.,  10,  853  (1904). 


62 


QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 


If  the  values  given  in  the  third,  fourth  and  fifth  columns  are 
computed  to  a  common  basis  of  1  gm.  of  deposited  metal,  then 
the  table  becomes  easier  to  comprehend  and  reads  as  follows: 


No. 

Grams  Pd 
deposited. 

zw. 

Zf 

zw  -  zf. 

Revolutions  per 
minute. 

1 

1 

7.14 

2.01 

5.13 

500 

2 

1 

7.42 

2.01 

5.41 

620 

3 

1 

4.07 

2.01 

2.06 

800 

4 

1 

2.6 

2.01 

0.59 

1000 

In  the  third  column,  the  shortening  of  the  time  required  with 
increase  in  the  number  of  revolutions  is  clearly  shown.*  If  the" 
deposition  of  the  metal,  from  the  first  instant  until  the  last  traces 
of  metal  were  removed  from  the  solution,  took  place  at  a  uniform 
velocity,  then  the  time  required  would  be  exactly  2.01  hours. t  It 
may  be  assumed  that  the  deposition  took  place  at  first  in  accord- 
ance with  Faraday's  law  and  this  rate  continued  as  long  as  the 
solution  remained  at  approximately  its  original  concentration. 
Gradually,  however,  the  solution  became  poorer  in  metal  and  the 
longer  time  required  is  due  to  the  fact  that  the  whole  of  the  current, 
from  the  beginning  to  the  end  of  the  experiment,  was  not  utilized 
for  depositing  the  metal.  By  subtracting  the  theoretical  time  re- 
quired from  the  actual  time  consumed,  the  values  given  in  the 
next  to  the  last  column  are  obtained,  and  these  values  show  that 
the  difference  between  the  theoretical  and  actual  periods  is  smaller, 
the  greater  the  velocity  of  revolution.  The  cause  of  this  more 
rapid  deposition,  theory  aside,  is  evidently  that  in  the  case  of 
intense  stirring  the  current  is  utilized  to  better  advantage  for  the 
desired  deposition  and  is  not  used  up  in  other  ways,  as  in  the 
liberation  of  hydrogen,  to  the  same  extent  as  with  a  stationary 
electrolyte  or  with  one  but  gently  stirred.  When  the  stirrer 
revolved  at  the  rate  of  1000  revolutions  per  minute  (Exp.  4, 
p.  61)  the  time  required  for  deposition  (2.6  hours)  was  nearest 
to  the  theoretical  value  (2.01  hours).  Thus  the  elimination  of 
hydrogen  was  avoided  throughout  the  entire  experiment,  and  this, 

*  Experiment  1  does  not  fall  quite  into  line  with  the  others.  Eitner  there 
is  a  mistake  here  or  else  the  current,  assumed  to  be  0.25  ampere,  was  not  per- 
fectly uniform  during  the  whole  period. 

t  To  get  this  value,  the  atomic  weight  of  Pd  is  taken  as  106.5,  its  valence 
2  and  the  current  strength  as  0.25  ampere. 


ELECTRO-ANALYSIS  WITH   MOVING  ELECTROLYTES         63 

as  has  been  explained  already,  is  a  most  desirable  condition  for 
obtaining  a  deposit  of  metal  in  compact  form.  If,  however,  the 
evolution  of  gas  is  prevented  entirely  in  the  rapidly  stirred  elec- 
trolyte, or,  in  other  words,  if  the  time  when  this  evolution  begins 
is  put  off  as  long  as  possible,  then  the  cause  for  this  behavior 
must  be  traced  to  the  fact  that  a  sufficient  quantity  of  metal  ions 
are  brought  in  contact  with  the  cathode,  and,  indeed,  with  such 
velocity  that  the  entire  electric  charge  of  the  cathode  is  neutralized 
by  metal  ions  alone;  thus  the  cathode,  we  may  say,  experiences 
no  requirement  for  other  ions  until  the  metal  is  all  deposited.  The 
following  explanation  of  the  processes  taking  place  during  elec- 
trolysis must  be  very  close  to  the  truth.  At  the  start,  when  the 
cathode  potential  becomes  high  enough  to  cause  deposition  of  the 
metal,  the  concentration  of  metal  ions  in  the  vicinity  of  the  cathode 
is  so  large  that  the  deposition  of  the  metal  takes  place  according 
to  Faraday's  law.  This  deposition  of  the  metal,  however,  often 
takes  place  faster  than  the  positively  charged  metal  ions  migrate 
toward  the  cathode.  The  most  desirable  condition  for  a  satisfac- 
tory electro-analysis  is  that  the  deposition  may  take  place  in  accord- 
ance with  Faraday's  law.  To  this  end,  it  is  requisite  that  an  excess 
of  metal  ions  should  be  present  all  the  time  at  the  cathode  and 
there  are  two  ways  of  accomplishing  this.  One  way  consists  in 
gradually  lessening  the  current  so  that  the  velocity  at  which  the 
metal  ions  are  discharged  is  constantly  less  than  that  of  the  migra- 
tion of  these  ions  toward  the  electrode.  This  method  of  working 
is  not  only  impractical  but  it  is  also  very  tedious. 

The  other  method  consists  in  artificially  bringing  the  ions  to 
the  cathode  with  a  velocity  greater  than  that  of  the  discharge  of 
the  ions.  This  is  brought  about  by  a  rapid  stirring  of  the  liquid. 
The  transference  of  the  ions  is  supported  naturally  by  diffusion; 
for  as  the  solution  in  the  vicinity  of  the  cathode  becomes  deficient 
in  ions  of  any  kind,  diffusion  seeks  to  make  the  concentration 
homogeneous  throughout  the  entire  solution.  In  some  cases  this 
suffices  to  satisfy  the  requirement  of  ions  at  the  cathode;  this  is 
the  case  with  very  complex  electrolytes.  Then  the  discharge  of 
the  metal  ions  takes  place  more  slowly  than  from  a  simple  elec- 
trolyte and  there  is  thus  always  an  excess  of  ions  at  the  cathode 
ready  to  be  discharged.  In  such  cases,  therefore,  the  duration  of 
the  electrolysis  cannot  be  shortened  materially  by  stirring  the 
electrolyte. 


64  QUANTITATIVE   ANALYSIS   BY  ELECTROLYSIS 

In  the  other  case,  when  the  discharge  of  the  metal  ions  takes 
place  at  a  very  high  velocity,  the  analysis  will  take  place  more 
rapidly  in  proportion  as  the  liquid  is  well  stirred. 

The  shortening  of  the  time  required  for  an  electro-analysis  by 
heating  the  electrolyte  can  be  explained  from  the  same  point  of 
view.  It  has  been  mentioned  on  page  20  that  the  conductivity 
of  the  electrolyte  is  increased  in  this  way.  It  is  also  true  that  the 
rate  of  diffusion  in  the  liquid  is  likewise  increased  and  thus  the 
effect  of  rapid  stirring  is  obtained,  at  least  in  a  measure.  In 
many  cases  the  combined  effect  of  heating  and  stirring  is  em- 
ployed in  electro-analysis.  The  effect  of  temperature  upon 
electrolytic  separations  in  complex  electrolytes  is  discussed  on 
page  94. 

The  important  reason  why  the  stirring  of  the  electrolyte  leads 
to  such  valuable  results  is  because  it  permits  the  use  of  a  much 
greater  current  strength  than  would  otherwise  be  possible.  This 
new  method  in  fact  permits  one  to  use  current  densities  that  would 
be  altogether  out  of  the  question  with  a  stationary  electrolyte,  if 
it  were  desired  to  obtain  deposits  free  from  sponginess. 

It  has  been  pointed  out,  on  page  36,  how  important  it  is  for 
certain  determinations  and  separations  to  measure  the  potential 
at  the  cathode.  Although  such  a  complication  of  the  analysis  is 
absolutely  necessary  in  some  cases,  still  the  great  advantage  of 
being  able  to  carry  out  some  analyses  within  ten  or  fifteen  minutes 
is  apparent  to  every  one  (see  the  article  on  Bismuth). 

A  brief  description  of  the  electrolytic  equipment  at  the  Aachen 
Institute  of  Technology  will  be  given  in  the  following  pages.* 
It  may  be  mentioned  at  the  start  that  experiments  carried  out  in 
this  laboratory  have  shown  that  it  is  absolutely  immaterial,  as 
regards  the  desired  result,  whether  the  solution,  is  stirred  by 
rotating  the  cathode,  the  anode,  or  both  together,  or  whether  an 
independent  stirrer  is  used. 

There  are,  then,  three  groups  of  electrode  pairs  used  for  rapid 
electro-analysis:  1.  Stationary  cathode,  rotating  anode.  2.  Sta- 
tionary anode,  rotating  cathode.  3.  Both  electrodes  stationary, 
independent  stirrer. 

To  the  first  group  belongs  the  platinum  dish  as  cathode  with 
anodes  of  various  types:  (a)  perforated,  flat  disk  (Fig.  21);  (b) 
perforated,  coirugated  disk  (Fig.  28);  (c)  spiral  (Fig.  15);  (d) 
*  A  fuller  account  can  be  found  in  Z.  Elektrochem.,  13,  181  (1907). 


ELECTRO-ANALYSIS  WITH   MOVING  ELECTROLYTES        65 


perforated  dish  electrode,  also  called  a  sieve  electrode  (Fig.  22). 
Sand's  gauze  electrode,  described  in  the  publication  cited  on  page 
42,  belongs  in  this  group.  Finally,  the  mercury 
cathode  used  by  Kollock  and  Smith  and  that 
used  by  Hildebrand  deserve  mention. 

The  second  group  is  represented  by  (a)  the 
rotating  platinum  crucible  as  cathode,*  (b)  the 
rotating  gauze  cathode  devised  in  this  labora- 
tory by  A.  Fischer  and  which  is  strengthened 
by  placing  it  over  a  hollow  porcelain  body 
(Fig.  29).  The  stem  of  the  latter  contains  a 
vertical  groove  in  which  a  somewhat  stronger 
platinum  wire  is  laid  loosely.  When  the  porcelain  stem  is  placed 
in  the  binding  post  of  the  apparatus,  this  wire  permits  the  passage 


FIG.  28. 


FIG.  29.  FIG.  30. 

of  the  current  to  the  gauze  with  the  aid  of  a  ring  of  platinum  foil 
against  which  the  platinum  wire  lies,  as  a  spring,  upon  the 
inside.  The  connection  between  the  ring  and  the  gauze  is 
furnished  by  two  fine  platinum  wires,  fastened  to  the  gauze, 
with  the  free  ends  placed  between  the  porcelain  stem  and  the 
platinum  ring  and  bent  over  on  the  outside.  The  fixed  stationary 
anode  used  with  this  cathode  is  made  of  platinum  wire  and 
*  Gooch  and  Medway,  Z.  anorg.  Chem.,  35,  414  (1903). 


66 


QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 


provided  with  windings   large   enough   to   inclose   the  cathode 
(Fig.  30). 

The  third  group  consists  of  two  stationary  electrodes.  In  this 
laboratory  A.  Fischer's  modification  of  Sand's  electrode  has  given 
satisfaction.  Sand's  electrodes  consisted  essentially  of  two  gauze 
coaxial  cylinders  of  which  the  inner  was  movable  and  served  as 
stirrer.  To  increase  the  stirring  effect,  the  inner  electrode  was 
provided  with  a  diametric  partition.  Sand's  aim  was  to  study 
the  cathode  potential  (i.e.,  the  difference  in  potential  between  the 
cathode  and  the  electrolyte),  during  the  electro-analysis,  and  his 
electrodes  were  arranged  with  this  end  in  view.  It  is  necessary, 
for  this  purpose,  that  the  auxiliary  electrode,  when  the  end  of  its 
capillary  tube  is  placed  in  the  neighborhood  of  the  cathode  (see 
Fig.  11,  p.  42),  should  show  the  exact  potential  of  the  cathode. 
This  is  actually  the  case  with  Sand's  electrodes;  the  current  lines 
from  the  anode  are  caught  so  completely  by  the  cathode  that 
the  capillary  tube  of  the  auxiliary  electrode  can  be  introduced  at 
almost  any  place  in  the  liquid  outside  the  cathode  without  there 


FIG.  31. 


FIG.  32. 


FIG.  33. 


FIG.  34. 


being  any  appreciable  difference  in  the  potential  values  that  are 
measured. 

The  only  objection  to  Sand's  apparatus  is  that  the  manner  of 
connecting  the  stirrer  to  the  motor  was  rather  more  complicated 
than  necessary.  A.  Fischer  simplified  matters  somewhat  by 


ELECTRO-ANALYSIS  WITH   MOVING  ELECTROLYTES        67 

making  the  stirrer  independent  of  the  anode;  at  the  same  time 
he  proved  experimentally  that  this  permitted  the  cathode  poten- 
tial to  be  measured  with  the  same  accuracy  as  with  Sand's  ar- 
rangement.* 


FIG.  35. 


FIG.  36. 


The  two  electrodes  A  and  K  (Figs.  31  and  32)  consist  of  fine- 
meshed  platinum  gauze.  To  prevent  contact  between  the  stems  of 
the  cylinders  when  K  is  placed  over  A,  there  is  placed  over  the  stem 
of  the  latter  a  piece  of  small  glass  tubing  G,  over  which  the  two  loops 
in  the  stem  of  K  will  slip.  Near  the  bottom  of  the  glass  tubing  G 
two  globules  of  glass  are  fused  to  it,  and  upon  these  rest  the  lower 
loop  on  the  stem  of  K.  To  prevent  any  contact  between  the  two 
cylinders,  the  cylinder  A  is  provided  with  four  small  pieces  of  glass 
rod  which  are  bent  over  at  the  top  and  bottom  to  hold  them  in 
place.  The  cylinder  K  slips  over  these  pieces  of  glass  with  slight 
friction,  so  that  all  parts  are  joined  to  one  another.  The  distance 
between  the  two  electrodes  is  about  3  mm. 

The  stirrer  R  (Fig.  33)  consists  of  three  or  four  thin  sheets  of 
*  Z.  Elektrochem.,  13,  469  (1907). 


68  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

glass,  placed  parallel  to  one  another,  3  or  4  mm.  apart,  and  fused 
together  at  the  top  (lattice  stirrer). 

The  thin  sheets  of  glass  are  not  placed  exactly  tangential  to 
the  circles  which  they  set  in  motion  but  are  inclined  slightly,  as 
a  glance  at  the  horizontal  projection  P  will  show.  The  plates  are 
fused  to  a  glass  rod  of  which  the  upper  end  is  covered  with  a  piece 
of  rubber  tubing  to  aid  in  connecting  it  with  the  shaft  of  the  motor 
(Fig.  35).  Plate  II  (back  of  the  book),  which  accompanies  the 
section  on  Bismuth,  shows  the  way  the  stirrer  is  placed  with 
reference  to  the  electrodes  shown  in  Figs.  31  and  32. 

Sand  found  that  his  form  of  rotating  anode  was  not  suitable  for 
use  in  the  deposition  of  metals  of  which  the  ions  in  solution  were 
likely  to  change  in  valence  during  the  electrolysis.  Thus,  for 
example,  he  was  unable  to  precipitate  the  last  traces  of  copper 
from  an  ammoniacal  solution.  These  difficulties  are  of  the  nature 
discussed  under  the  deposition  of  copper  from  acid  solutions  (p. 
121).  Just  as  in  that  case  the  high  temperature  is  favorable  for 
carrying  out  the  reversible  reaction,  Cu++  +  Cu  ^±  2  Cu+,  so 
in  this  case  the  violent  stirring  serves  to  effect  the  intimate  inter- 
change of  the  products  of  the  oxidation  at  the  anode  with  the 
products  of  reduction  at  the  cathode,  and  this  tends  either  to  pro- 
long the  analysis  or  to  prevent  the  complete  deposition  of  the  metal. 
To  obviate  this  difficulty,  Sand  had  a  special  anode  made  having 
a  small  platinum  surface  and  small  stirring  face.  A.  Fischer 
accomplishes  the  same  end  by  using  a.  less  effective  stirrer  and 
keeping  the  electrodes  the  same.  This  stirrer  consists  of  a  piece 
of  glass  rod  made  into  the  shape  of  a  helix  (Fig.  34,  p.  66). 

The  apparatus  designed  by  the  author's  assistant,  A.  Fischer 
(Figs.  35  and  36),  is  different  from  others  that  have  been  devised 
for  the  same  purpose,  inasmuch  as  the  motor,  which  serves  to  drive 
the  stirrer,  is  fastened  to  the  upper  end  of  an  upright,  and  its 
motion  is  transferred  to  the  electrode,  or  other  stirrer,  by  means 
of  a  flexible  steel  shaft  (a  piece  of  steel  wire  wound  into  a  helix). 
The  motor  is  driven  by  power  furnished  from  the  lighting  circuit 
with  a  potential  of  110  volts  and  is  independent  of  the  current 
used  for  the  electrolysis.  During  the  electrolysis,  the  vessel  is 
covered,  to  prevent  loss  by  spattering,  with  a  watch  glass  which 
has  a  small  perforation  in  the  middle,  to  permit  the  wire  stem  of 
the  anode  to  pass  through  it.  When  the  anode  is  raised,  the  watch 
glass  is  lifted  with  it.  Through  another  perforation  at  one  side 


ELECTRO-ANALYSIS  WITH  MOVING  ELECTROLYTES       69 

of  the  watch  glass,  a  thermometer,  likewise  attached  to  the  up- 
right, can  be  introduced.     If  it  is  desired  to  heat  the  electrolyte 


during  the  analysis,  a  piece  of  asbestos  paper  is  placed  on  a  ring  a 
little  below  the  dish  and  a  small  flame  is  placed  below  the  asbestos, 


70 


QUANTITATIVE   ANALYSIS  BY  ELECTROLYSIS 


so  that  the  dish  is  heated  very  uniformly  by  means  of  the  hot  air 
arising  from  the  asbestos. 

Figure  36  shows  the  apparatus  fitted  up  for  use  with  a  rotating 
cathode  (Fig.  29,  p.  65).  In  this  case,  the  glass  vessel  contain- 
ing the  electrolyte  is  provided  with  a  stopcock  at  the  bottom  to 
facilitate  the  final  washing  of  the  deposit. 

To  permit  several  analyses  being  carried  out  at  the  same  time 
without  any  interference,  the  working  bench  shown  in  Fig.  37  is 
arranged  as  follows: 

In  the  closet  below  the  bench  is  a  battery  of  accumulators 
consisting  of  24  cells.  The  battery  rests  upon  a  board,  which  is  on 
castors  so  that  it  can  be  withdrawn  easily  in  case  it  is  necessary 
to  make  repairs. 

All  the  cells  are  kept  connected  in  series  and  are  charged  from 
the  electric-lighting  circuit,  using  a  wire  rheostat  (Fig.  38  and 
L.R.  at  the  lower  left-hand  cor- 
ner of  Plate  I).  This  wire  re- 
sistance is  placed  upon  a  marble 
slab  at  one  end  of  the  bench  and 
upon  the  slab  is  a  switch  for 
turning  the  current  on  and  off, 
also  a  rheostat  handle  for  regu- 
lating the  resistance  and  an  am- 
meter (Fig.  38  or  lower  left-hand 
corner  of  Table  I). 

The  bench  is  fitted  up  with 
six  working  places  and  thus  four 
accumulator  cells  are  furnished 
for  each  working  place. 

(Plate  I:  Group  I  over  1,  2,  3, 
4;  Group  II  over  5,  6,  7,  8,  etc.) 

To  start  an  analysis  a  number 
of  operations  are  necessary.  At 
the  back  of  the  bench,  next  the 
wall,  is  a  top  piece  upon  which 
are  fastened  the  socket  for  the 
motor  connection  and  the  bind- 
ing posts  for  making  connection 
with  the  electrolytic  cell  (see  Fig. 
39).  The  contact  plug  fastened 
to  the  end  of  the  wires  leading 


FIG.  38. 


ELECTRO-ANALYSIS  WITH  MOVING  ELECTROLYTES      71 


to  the  motor  is  pushed  into  the  socket  and  the  wires  from  the 
electrodes  are  inserted  into  the  -f-  and  —  posts  of  cell  connec- 
tion. Upon  the  front  of  the  bench,  over  the  closet  doors,  is  at- 
tached a  marble  slab,  at  the  right-hand  side  of  which  is  the  rheo- 
stat handle  M.R.  (Fig.  39)  for  turning  the  motor  on  and  off  as 
well  as  for  regulating  its  velocity;  this  can  be  varied  between 
250  and  1600  revolutions  per  minute. 

The  line  drawing  in  Plate  I  shows  how  the  motor  is  connected 
to  the  middle  wire  and  positive  outside  wire  of  the  three-wire 
system  from  the  electric-lighting  plant. 

The  handle  A.R.,  on  the  left-hand  side  of  the  marble  slab 
(Fig.  39  and  Plate  I),  is  for  turning  the  current  on  and  off  from 
the  storage  battery,  from  which  the  current  used  for  the  electro- 
lytic cell  is  obtained,  and  this  handle  also  serves  for  varying  the 
resistance  as  indicated  in  Plate  I. 

For  measuring  the  strength  of  the  current  passing  through  the 
cell,  and  for  measuring  the  drop  in  potential  of  the  current  in 
passing  through  the  cell,  there  is  only  one  ammeter  and  one  volt- 
meter for  the  six  working  places.  These  two  instruments,  as 
shown  in  Fig.  37,  are  near  the  wall  upon  uprights  and  fixed  so  that 
they  can  be  revolved  and  read  from  each  working  place.  As  can 


Hotor  Connection 


Fuse  for  Cell 


Fuses  for  Motor 


FIG.  39. 

be  seen  in  Fig.  37,  or  more  distinctly  in  the  line  drawing  of  Fig.  39, 
there  is  a  plate  in  the  middle  of  the  marble  slab,  carrying  a  double- 
throw  switch  in  the  center.  If  this  switch  is  thrown  down,  in 
the  direction  of  the  lower  arrow,  shown  in  Fig.  39,  the  ammeter 
is  placed  in  the  electrolytic  circuit  of  this  bench.  If  the  switch 


72 


QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 


is  pushed  upward,  in  the  direction  of  the  upper  arrow  (Fig.  39), 
then  the  voltage  of  the  current  can  be  determined.  After  each 
reading,  the  switch  must  be  returned  to  its  central  position,  as 
otherwise  it  is  impossible  to  make  a  reading  from  any  other  bench. 

The  electrical  connections  are  so  arranged  that  work  can  be  car- 
ried out  at  each  bench  not  only  with  the  8  volts  from  the  four  stor- 
age cells  under  it  but,  if  desired,  it  is  possible  to  take  the  current  from 
the  four  neighboring  storage  cells  and  thus  work  can  be  performed 
with  16  volts.  This  adjustment  of  the  current  is  effected  with  the 
key  U,  which  is  near  the  bottom  of  the  marble  slab  at  the  middle. 
When  the  switch  is  turned  to  the  point  marked  8,  then  work  at 
that  bench  is  carried  out  with  8  volts,  and  when  it  is  desired  to  use 
16  volts,  the  switch  is  turned  to  the  position  marked  16.  The 
connections  are  made  so  that 
even  in  the  latter  case  there  is 
no  interference  with  work  at  the 
neighboring  bench. 

The  sketches  shown  in  Figs. 
41,  42  and  43  show  how  the 
switch  U  serves  in  its  two  posi- 
tions to  make  connections  with 
the  storage  batteries  for  two 
neighboring  electrolytic  cells. 

Figure  40  shows  the  connec- 
tions at  Places  I  and  II  when 
both  their  keys  are  placed  at 
8  volts.  It  is  evident  from  this 
sketch  that  the  wire  ab  plays  the 
same  part  as  the  neutral  wire  in 
a  three- wire  lighting  system. 

If  the  two  keys  at  Places  III 
and  IV  are  both  turned  to  16 
volts,  then  the  way  the  connec- 
tions are  made  is  shown  in  Fig. 
41  (see  also  Plate  I). 


I    I 
FIGS.  40,  41  and  42. 


Figure  42  represents  the  connections  when  the  work  at  Place  V 
is  carried  out  with  8  volts  and  at  Place  VI  with  16  volts. 

Thus,  six  or  less  students  can  all  work  independently  at  any  time, 
using  either  8  or  16  volts  without  any  interference  with  one  another. 

Not  only  the  voltage  but  also  the  strength  of  the  current  can 


ELECTROLYSIS   BY   MEANS  OF   MAGNETIC  STIRRING     73 

be  varied  within  wide  limits.  If,  for  example,  the  student  at  any 
one  of  the  places  is  working  with  a  battery  current  of  10  amperes, 
he  can  at  any  time  get  an  additional  6  amperes  from  the  lighting 
system,  and  thus  carry  out  the  work  with  16  amperes,  by  turning 
the  control  handle  K  at  the  charging  circuit  (Fig.  38). 

A  cheap  and  practical  arrangement  for  carrying  out  rapid 
electro-analyses  has  been  described  by  A.  M.  Fairlee  and  A.  J. 
Bone.*  Their  outfit  is  arranged  especially  for  the  determination 
of  copper,  and  eight  determinations  can  be  carried  out  at  one  time 
with  the  use  of  only  one  motor. 


FIG.  43. 


Rapid  Electrolysis  by  Means  of  Magnetic  Stirring. 

E.  A.  Ashcroft,f  in  studying  the  electrolysis  of  fused  salts, 
found  that  he  was  able  to  get  a  very  favorable  stirring  of  the 
electrolyte  by  surrounding  the  decomposition  cell  with  a  spool  of 
wire,  through  which  the  current  used  ^for  the  electrolysis  flowed. 

*  Electrochem.  Met.  Ind.,  6,  19,  58  (1908). 
f  Ibid.,  4,  143  (1906). 


74 


QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS 


F.  C.  Frary  *  applied  the  same  principle  to  the  electro-analysis 
of  solutions  and  devised  the  following  two  forms  of  apparatus. 

The  apparatus  shown  in  Fig.  43  consists  of  a  spool  of  insulated 
copper  wire,  1.5  mm.  in  diameter,  having  a  total  resistance  of  about 
1  ohm.  The  wire  is  coiled  around  a  cylinder  of  sheet  copper  which 
is  made  to  hold  the  beaker  in  which  the  electrolysis  is  to  take  place. 
The  spool  is  covered  with  a  sheet-iron  mantle,  rests  upon  an  iron 
base,  and  contains  inside,  at  the  bottom,  a  thick,  hollow  iron 
cylinder  as  core;  the  beaker  rests  upon  this  core.  By  this  arrange- 
ment the  magnetic  field  in  which  the  beaker  rests  is  strengthened 


FIG.  44. 

and  concentrated  above  the  hollow  iron  cylinder.     The  direction 
of  the  magnetic  lines  of  force  is  vertical. 

The  electrodes  shown  in  Figs.  24  and  25  on  page  60  are  used 
with  this  apparatus  and  between  them  the  electricity  flows  hori- 
zontally and  radially.  If,  now,  the  entire  electrolyte  is  imagined 
to  consist  of  separate  radial  threads,  then  each  thread  forms  a 
conductor  through  which  the  current  flows  and  the  direction  of 
the  current  is  perpendicular  to  the  magnetic  lines  of  force  passing 

*  J.  Am.  Chem.  Soc.,  29,  1592  (1907). 


ELECTROLYSIS   BY   MEANS  OF  MAGNETIC  STIRRING     75 

through  the  solution.  Consequently,  there  acts  upon  the  radial 
threads  of  liquid  a  horizontal  force  perpendicular  to  them  and  as 
a  result  the  liquid  is  rotated  about  the  axis  of  the  apparatus. 

The  current  used  for  the  electrolysis  will  serve  for  exciting  the 
induction  current,  and,  in  that  case,  the  spool  and  the  electrolytic 
cell  are  connected  with  one  another  in  series,  or,  if  more  convenient, 
an  independent  current  may  be  used  in  the  coil  (see  below). 

Frary  was  able  with  a  current  of  6  to  7  amperes  during  the  first 
five  minutes,  and  afterwards  of  4  amperes,  to  deposit  0.85  gm.  of 
copper  quantitatively  in  15  minutes.  The  electrolyte  used  was 
100  cc.  of  copper-sulphate  solution  acidified  with  10  drops  of  con- 
centrated sulphuric  acid.  The  potential  between  the  electrodes 
was  about  8  volts  during  the  last  part  of  the  operation. 

Another  form  of  apparatus  used  by  Frary  (Fig.  44)  depends 
upon  the  use  of  a  mercury  cathode.  The  magnetic  field  is  pro- 
duced here  between  the  two  poles  of  a  vertically  placed  electro- 
magnet; one  pole  is  formed  by  the  upper  end  of  the  iron  core  which 
projects  from  the  middle  of  the  spool,  and  the  other  pole  of  the 
electromagnet  is  obtained  by  uniting  the  iron  core  with  the  iron 
base  and  the  iron  sides  of  the  frame  in  the  upper  annular  part  of 
the  frame  which  surrounds  the  projecting  core.  The  magnetic 
lines  of  force  in  this  case  run  in  a  horizontal  radial  direction  be- 
tween the  iron  core  and  the  annular  upper  part  of  the  frame. 

The  bottom  of  the  electrolyzing  vessel  is  raised  at  the  middle 
so  that  it  looks  almost  as  if  the  bottom  had  been  pushed  up  at  the 
middle  by  means  of  an  inverted  test  tube.  The  hollow  thus  formed 
fits  over  the  projecting  iron  core  and  the  solution  itself  is  contained 
in  an  annular  space.  Three  short  platinum  points  are  fused  into 
the  bottom  of  the  vessel  and  these  rest  upon  a  copper  disk.  In 
this  way  a  connection  is  made  between  the  mercury  in  the  vessel 
and  the  copper  disk  and  the  latter  is  connected  with  the  negative 
pole  of  the  electrolyzing  circuit  by  means  of  an  insulated  wire 
which  is  introduced  through  the  frame  of  the  apparatus.  The 
anode  consists  of  a  platinum  wire  wound  into  a  spiral;  it  is  marked 
+  in  the  picture.  The  current,  therefore,  flows  vertically  through 
the  cell,  and  since,  as  mentioned  above,  the  magnetic  lines  of  force 
flow  in  a  radial  direction,  the  proper  conditions  are  provided  for 
a  movement  of  the  electrolyte.  With  this  apparatus,  the  motion 
of  the  electrolyte  is  much  more  rapid  than  with  the  apparatus 
first  described,  for  the  simple  reason  that  the  magnetic  lines  of 


76  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

force  are  concentrated  more  by  the  iron  core.  The  electro- 
magnet, from  which  the  copper  disk  is  separated  by  another  disk 
of  insulating  material,  is  excited  by  means  of  a  separate  current 
which  is  introduced  at  the  two  bottom  binding  posts  shown  at  the 
right-hand  side  of  Fig.  44. 

Using  this  apparatus,  Frary  was  able  to  deposit  0.1  gm.  of  iron 
in  10  minutes  with  a  current  of  4  amperes,  which,  in  this  case,  also 
flowed  through  the  spool  of  the  electromagnet. 

Such  an  apparatus  has  the  advantage  over  mechanical  stirring 
of  not  being  as  expensive  and  it  requires  less  supervision.  Time 
alone  will  show  which  method  proves  the  better  in  practice. 

In  the  author's  laboratory  the  former  of  the  two  types  of  ap- 
paratus designed  by  Frary  (Fig.  43)  has  been  tested  carefully. 

When  we  remember  that  the  rate  at  which  the  electrolyte  is 
stirred  depends  not  only  upon  the  strength  of  the  magnetic  field 
but  also  upon  the  strength  of  the  current  used  for  the  electrolysis, 
it  is  obvious  that  the  method  is  limited  in  its  application;*  for, 
whenever  the  determination,  or  separation,  of  a  metal  takes  place 
at  a  constant  voltage,  the  current  strength  toward  the  end  of  the 
operation  sinks  to  a  very  low  value  and  that  component  of  the 
stirring  force  which  depends  upon  the  analyzing  current  becomes 
too  small  to  produce  the  desired  stirring  even  although  the  mag- 
netic field  is  very  strong.  This  is  just  the  time,  on  account  of  the 
very  low  concentration  of  metal  ions  remaining  in  solution,  when 
the  solution  should  be  stirred  most  effectively.  It  is  possible,  to  be 
sure,  to  increase  the  independent  induction  current  in  the  spool 
but  the  size  of  the  spool  places  a  limit  upon  the  extent  to  which 
this  can  be  done.  Frary  gives  the  resistance  of  the  spool  as  1  ohm 
and  the  current  as  5  amperes.  Probably  6  amperes  of  current 
would  be  all  that  the  coil  could  stand  and  if  more  were  used  the 
insulation  of  the  wires  would  be  likely  to  melt. 

A  current  of  6  amperes  would  have  to  be  used  in  the  coil,  for 
example,  in  separating  copper  from  zinc;  because,  to  deposit 
copper  free  from  zinc,  the  current  used  for  the  analysis  should  not 
exceed  3.5  amperes.  If  the  magnetizing  current  were  less  than 
6  amperes,  there  would  not  be  enough  stirring  to  keep  the  zinc 
in  solution. 

If  the  strength  of  the  current  must  be  kept  low,  it  will  often 

*  A.  Fischer,  Z.  Elektrochem.,  14,  35  (1908). 


ELECTROLYSIS  BY   MEANS  OF  MAGNETIC  STIRRING     77 

happen  that  the  stirring  is  insufficient  to  obtain  a  good  deposit  of 
metal. 

Although  the  heating  effect  in  the  coil  is  favorable  to  the  elec- 
trolysis in  most  cases,  yet  sometimes,  as  in  the  deposition  of  zinc 
from  acid  solutions,  this  proves  a  disadvantage.  In  such  cases,  a 
narrower  beaker  should  be  used  in  the  first  apparatus  and  the 
beaker  should  be  surrounded  by  a  coil  of  lead  pipe  through  which 
cold  water  flows. 

Finally,  another  objection  that  may  be  raised  is  the  fact  that 
the  current  consumption  is  considerable  in  those  cases  where  the 
magnetizing  current  is  made  stronger  than  the  current  used  for 
the  analysis. 

In  electrolytic  work  where  the  deposition  can  be  effected  with 
high  current  densities,  the  Frary  apparatus  has  proved  very 
satisfactory.  To  show  this,  the  following  experimental  results 
will  be  given. 

i.   Copper. 

Electrolyte  contained 1  cc.  nitric  acid  (sp.  gr.  1.2). 

Strength  of  current  for  the  analysis 3 . 8  to  4  amperes. 

Strength  of  the  magnetizing  current 4. 8  to  5  amperes. 

Temperature Boiling. 

Time  required 20  minutes. 

Result:   quantitative  deposition,  deposit  a  beautiful  pink. 

2.  Iron. 

Electrolyte  contained 5  to  6  gms.  ammonium  oxalate 

to  about  0.1  gm.  iron. 

Strength  of  current  for  the  analysis 4  amperes. 

Strength  of  the  magnetizing  current 4.8  amperes. 

Initial  temperature 50°  to  60°. 

Final  temperature 70°  to  75°. 

Time  required 30  minutes. 

Result:  quantitative  deposition,  deposit  steel  gray. 

3.   Nickel. 

Electrolyte  contained 1.5  gms.  ammonium  sulphate, 

25  cc.  ammonia  (sp.  gr.  0.91) 
to  about  0.2  gm.  of  nickel. 

Strength  of  current  for  the  analysis 5  amperes. 

Strength  of  the  magnetizing  current 4.8  amperes. 

Initial  temperature 70°. 

Final  temperature 80°. 

Time  required 20  minutes. 

Result:  quantitative  deposition,  deposit  light  colored  and  dense. 


78  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

4.    Tin. 

Electrolyte  contained 16  gms.  of  ammonium  sulphide 

solution  to  1  gm.  of  zinc-am- 
monium chloride. 

Strength  of  current  for  the  analysis 3  to  3 . 5  amperes. 

Strength  of  the  magnetizing  current 5  amperes. 

Initial  temperature 50°  to  60°. 

Final  temperature 70°  to  75°. 

Time  required 20  minutes. 

Result:  quantitative  deposition,  deposit  bright  and  lustrous. 

5.    Separation  of  Copper  from  Zinc. 

The  quantitative  deposition  of  the  copper  was  successful  at  the  end  of 
20  minutes,  using  a  current  of  3.5  amperes  and  a  magnetizing  current  of 
6  amperes,  with  the  other  experimental  conditions  the  same  as  under  1. 

G.  L.  Heath  of  the  Calumet  and  Hecla  Works  in  Michigan 
has  had  considerable  practical  experience  with  apparatus  similar 
to  that  of  Frary  and  recommends  it  highly.*  He  uses  it  in 
two  sizes;  one  large  enough  to  accommodate  a  lipless  beaker 
of  about  6  cm.  diameter  and  300  cc.  capacity  which  is  suitable 
for  the  electrolysis  of  samples  weighing  5  gms.  and  the  other 
large  enough  to  take  a  500-cc.  beaker  and  electrolyze  samples 
weighing  up  to  50  gms. 

The  use  of  such  large  weights  of  metal  is  advocated  simply 
in  order  to  obtain  more  representative  samples  and  to  obtain 
solutions  free  from  copper  which  will  contain  appreciable  quan- 
tities of  impurities  present  only  to  small  fractions  of  1  per  cent. 

For  the  smaller  apparatus,  a  copper  cylinder  is  made  of  7-cm. 
diameter,  using  metal  which  is  about  -^  in.  thick.  This  ie  wound 
with  500  turns  of  No.  13,  B.  &  S.  gauge  f  magnet  wire.  The 
cylinder  at  the  top  and  bottom  is  brazed  to  water-tight  joints  with 
thin  plates  of  soft  steel  which  complete  the  spool  holding  the  coil 
of  wire.  A  hole  is  bored  in  the  upper  steel  plate  of  a  size  equal 
to  the  inner  diameter  of  the  cylinder  and  a  1-in.  hole  is  bored 
through  the  bottom  plate  to  provide  ventilation  or  to  permit 
the  insertion  of  a  stopper  and  glass  tubes  for  water  cooling. 

Gauze  cathodes  weighing  16  to  17  gms.  and  having  about  17 
meshes  to  the  linear  centimeter  are  used  with  the  apparatus. 

*  J.  Ind.  Eng.  Chem.,  3,  77  (1911). 

t  This  is  the  standard  gauge  in  the  United  States  at  this  time.  The  initials 
stand  for  Brown  and  Sharpe. 


ELECTROLYTIC   DETERMINATIONS  AND  SEPARATIONS  79 

At  the  Calumet  and  Hecla  Works  a  current  of  4.5  amperes  is 
used  for  the  electrolysis  and  in  the  coil.  Five  gms.  of  copper 
are  deposited  in  about  2.5  hours. 

The  larger  apparatus  is  made  in  the  same  way  except  that  the 
diameter  of  the  cylinder  is  larger. 

As  regards  the  directions  for  carrying  out  a  rapid  electro- 
analysis,  it  is  impossible  to  make  them  broad  enough  to  cover  all 
conditions  that  are  likely  to  arise.  The  best  that  can  be  done,  at 
present,  is  to  state  in  the  form  of  tables  some  of  the  conditions 
under  which  good  results  have  been  obtained ;  it  is  usually  possible, 
then,  to  derive  from  the  tables  the  data  necessary  to  cover  any 
special  case.  Such  tables  are  given  in  the  section  devoted  to  the 
determination  of  the  individual  metals;  in  each  case  the  condi- 
tions for  carrying  out  the  analysis  with  the  same  electrolyte  by 
the  ordinary  slow  method  are  given  first.  The  experimental 
conditions  were  either  worked  out  in  the  author's  laboratory 
or  tested  there,  and  the  name  of  the  author  is  given  in  each 
case. 

The  foregoing  portions  of  this  book  contain  a  description  of  the 
various  forms  of  apparatus  which  are  used  for  carrying  out  electro- 
analyses  and  before  passing  on  to  that  part  of  the  book  which 
treats  of  the  directions  for  carrying  out  the  work,  it  is  necessary  to 
discuss  a  few  more  things  of  a  general  nature. 

The  purpose  of  electro-analysis  is  not  merely  to  determine  the 
individual  metals  but  it  serves  also  to  separate  certain  metals  from 
others. 


Electrolytic  Determination  of  a  Metal  and  Electrolytic 
Separations. 

The  only  metals  which,  up  to  the  present  time,  have  been 
deposited  satisfactorily  as  such  by  the  action  of  the  electric  cur- 
rent upon  solutions  are:  Zn,  Cd,  Tl,  Sn,  Bi,  Sb,  Fe,  Co,  Cu,  Hg, 
Ag,  Pd,  Pt  and  Au.  Thallium,  to  be  sure,  can  be  deposited  as 
metal  but  cannot  be  weighed  in  this  form  on  account  of  the 
extreme  readiness  with  which  it  undergoes  oxidation  (see  section 
on  Thallium).  The  remainder  of  the  above  metals  can  be  weighed 
as  such  upon  the  platinum  cathode. 

Manganese  and  lead  are  deposited  as  peroxides  upon  the  anode 
and  molybdenum  and  uranium  as  oxides  upon  the  cathode.  The 


80  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

alkali  and  alkaline-earth  metals  may  be  deposited  as  amalgams 
on  a  mercury  cathode  and  weighed  in  this  form. 

The  fact  that  the  most  suitable  solution  from  which  an  electro- 
analysis  can  be  made  is  not  the  same  with  different  metals,  so  that 
general  directions  can  be  given  which  will'  apply  in  all  cases,  has 
already  been  mentioned  on  page  44.  Similarly,  there  are  no 
general  rules  governing  the  deposition  of  a  second  metal  after 
the  first  has  been  quantitatively  deposited. 

The  underlying  principle  upon  which  methods  of  separation 
rest  is  to  remove  one  metal  at  a  low  potential  and  then,  when  all 
of  this  metal  has  been  deposited,  to  deposit  the  second  metal  by 
raising  the  potential.  Sometimes  simple  acid  solutions  are  suit- 
able but  at  other  times  it  is  necessary  to  provide  the  requisite 
differences  in  decomposition  potential  by  transforming  the  metals 
into  complex  salts  of  such  a  type  that  one  of  the  metals  enters 
into  a  stronger  complex,  or  one  decomposed  with  greater  difficulty 
than  the  other.  In  many  instances  it  is  necessary  to  change  the 
original  acid  or  complex  solution  into  some  other  kind  of  solution 
before  the  second  metal  can  be  deposited. 

According  to  the  decomposition  potentials  of  the  metals  given 
on  page  31  et  seq.,  it  might  be  imagined  that  it  is  merely  necessary 
to  increase  the  voltage  of  the  bath  above  these  values  to  effect  the 
deposition  of  the  various  metals.  In  practice,  however,  one  of  the 
chief  considerations  is  the  nature  of  the  deposit  formed;  the  metal 
must  not  only  be  deposited  pure  but  it  must  adhere  firmly  to  the 
electrode.  That  the  evolution  of  hydrogen  acts  as  a  disturbing 
factor  has  already  been  mentioned  on  page  53  and  elsewhere. 
Now,  as  is  well  known,  practically  all  solutions  contain  more  or 
less  hydrogen  ions.  Thus  water  itself  is  slightly  dissociated  into 
hydrogen  and  hydroxyl  ions.  In  many  cases,  however,  the  con- 
centration of  hydrogen  ions  is  kept  fairly  high  by  adding  acid  to 
the  electrolyte.  If  the  decomposition 'potential  of  the  metal  ion 
lies  close  to  that  of  the  hydrogen  ion,  even  though  it  is  lower, 
there  is  considerable  danger  of  hydrogen  ions  being  discharged  and 
this  danger  increases  as  the  solution  becomes  poorer  in  metal  ions, 
during  the  progress  of  the  electrolysis.  It  is  a  matter  of  common 
observation  that  at  the  beginning  of  an  electrolysis  there  is  abso- 
lutely no  evolution  of  hydrogen,  but  after  a  little  while  the  hydro- 
gen gas  begins  to  appear  and  the  evolution  increases  constantly  as 
the  work  proceeds.  It  is  necessary,  therefore,  in  order  to  obtain 


ELECTROLYTIC  DETERMINATIONS  AND  SEPARATIONS  81 

good  deposits,  to  carry  out  the  operation  so  that  the  evolution  of 
hydrogen,  if  not  altogether  prevented,  is  put  off  as  long  as  possible 
until  a  fairly  strong  deposit  has  been  formed  which  is  less  affected 
by  the  gas.  To  understand  better  the  conditions  under  which  a 
simultaneous  discharge  of  two  different  ions  takes  place,  let  us 
leave  hydrogen  out  of  consideration  for  the  time  being  and  assume 
that  we  have  a  solution  of  two  metals,  such  as  zinc  and  cadmium, 
present  in  approximately  equal  concentration  at  the  start  of  an 
electrolysis.  If  the  potential  between  the  electrodes  is  increased 
gradually,  the  time  soon  comes  when  one  of  the  metals  begins 
to  deposit  and  this  is  when  the  decomposition  potential  of  that 
metal  has  been  reached;  or,  since  the  decomposition  potential  is 
an  electromotive  force  composed  of  cathode  potential  and  anode 
potential,  it  is  more  accurate  to  say  that  the  deposition  of  one  of 
the  metals  starts  when  the  requisite  cathode  potential  is  reached. 
Since  cadmium  has  a  lower  decomposition  potential  than  zinc, 
at  first  only  cadmium  is  deposited  and  as  a  result  the  solution 
gradually  becomes  poorer  in  cadmium  ions.  Now  it  has  already 
been  explained  that  the  current  strength  is  proportional  to  the 
quantity  of  ions  neutralized  at  the  electrode  and  consequently  the 
current  must  necessarily  weaken  unless  sufficient  cadmium  ions 
are  present  in  the  vicinity  of  the  cathode.  If  the  current  strength 
is  kept  the  same,  then  the  voltage  of  the  current  gradually  increases 
and,  as  a  result  of  this,  the  decomposition  potential  of  zinc  ions 
is  reached  before  all  the  cadmium  is  deposited  and  then  both  zinc 
and  cadmium  are  precipitated  together. 

The  above  representation  holds  equally  true  if  we  substitute 
hydrogen  ions  for  the  zinc  ions.  The  simultaneous  discharge  of 
metal  ions  and  of  hydrogen  ions  is  not  the  only  part  that  the  latter 
play  in  electro-analysis. 

As  was  shown  on  page  32  and  as  is  apparent  from  the  above  ex- 
ample with  cadmium  and  zinc,  the  cathode  potentials  of  different 
metals  are  unlike  and  when  two  or  more  kinds  of  metal  ions  are 
present  in  a  solution,  the  deposit  first  obtained  will  be  of  the  metal 
having  the  lowest  decomposition  potential.  Hydrogen,  in  respect 
to  the  more  important  metals,  occupies  an  intermediate^  position  in 
the  potential  series.  The  order  is  as  follows:  Mg,  Al,  Mn,  Zn, 
Fe,  Cd,  Co,  Ni,  Pb,  Sn,  H,  As,  Bi,  Cu,  Sb,  Hg,  Ag,  Pd,  Pt,  Au. 
Since  those  metals  to  the  left  of  hydrogen  have  a  higher  poten- 
tial than  hydrogen  while  those  at  the  right  have  a  lower  potential, 


82  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

it  is  plain,  from  what  has  been  said,  that  the  metals  on  the  right 
are  precipitated  more  readily  and  those  on  the  left  less  readily 
than  hydrogen.  The  further  conclusion  that  zinc  and  cadmium 
are  deposited  only  after  all  the  hydrogen  ions  are  discharged  (i.e., 
practically  not  at  all)  is  not  quite  true.  This  fact  is  due  to  the  so- 
called  overvoltage  of  hydrogen  toward  different  metals.  If,  namely, 
hydrogen  requires  a  certain  low  voltage  in  order  to  be  set  free 
when  in  contact  with  so-called  platinized  *  platinum,  it  requires 
a  higher  voltage  to  discharge  hydrogen  ions  when  in  contact  with 
polished  platinum,  or  with  cadmium,  zinc  and  other  metals. 
This  excess  voltage,  which  not  only  varies  with  different  metals 
but  also  depends  upon  whether  the  surface  of  the  metal  is  rough 
or  smooth,  upon  the  temperature,  and  upon  whether  the  current 
density  is  high  or  low,  is  called  the  overvoltage  of  hydrogen  toward 
the  metal  in  question.  In  other  words,  it  is  harder  for  hydrogen 
ions  to  be  set  free  when  in  contact  with  some  metals  than  when 
in  contact  with  others  and  this  is  the  reason  why  zinc,  and  cad- 
mium can  be  deposited  by  the  electric  current  from  a  solution 
containing  very  dilute  acid  (see  following  section).  A  number 
of  theories  have  been  advanced  to  account  for  overvoltage.  Nernst 
has  assumed  that  when  ionic  hydrogen  is  discharged  it  is  in  the 
form  of  monatomic  hydrogen  which  at  a  slower  rate  is  converted 
into  diatomic  hydrogen  molecules.  Newbury  has  regarded 
hydrogen  overvoltage  as  due  chiefly  to  the  formation  of  metallic 
hydrides  with  higher  solution  tensions  than  that  of  hydrogen. 
More  recently,  however,  Maclnnes  and  Adler  f  have  argued 
convincingly  that  this  overvoltage  is  due,  primarily,  to  a  layer 
of  supersaturated,  dissolved  hydrogen  in  the  electrolyte  surround- 
ing the  cathode.  If  the  electrode  can  adsorb  large  hydrogen  gas 
nuclei  to  start  the  formation  of .  bubbles,  the  supersaturation 
cannot  rise  to  high  values  and  the  electrode  will  have  a  low  over- 
voltage.  Metals  with  small  adsorptive  powers  hold  small  nuclei 
and  have  high  overvoltages.  Maclnnes  and  Adler  have  obtained 
experimental  evidence  of  the  presence  of  such  nuclei,  and  have 
tested  their  theory  in  several  ways. 

*  Platinized  platinum,  i.e.  platinum  coated  with  platinum  black,  is  ob- 
tained by  electrolyzing  a  three  per  cent  solution  of  chloroplatinic  acid,  to 
which  one-fortieth  of  a  per  cent  of  lead  acetate  has  been  added.  By  using 
a  current  such  that  there  is  only  a  slight  evolution  of  gas,  the  platinum 
cathode  becomes  sufficiently  coated  within  a  few  minutes. 

f  J.  Am.  Chem.  Soc.  41,  194  (1919). 


THE   DEPOSITION   OF   METALS  83 


The   Deposition   of   Metals   from   Simple   and   from   Complex 

Electrolytes. 

H.  Danneel*  raised  the  following  questions  in  1903:  "What 
can  we  accomplish  by  an  electrolysis  and  what  do  we  know 
about  electrolysis?"  It  is  quite  proper  to  raise  such  questions 
now  and  then  and  to  look  back  over  the  path  which  investigation 
has  followed  in  the  field  of  jlectro-analysis.  It  was  not  long  ago 
when  our  knowledge  of  electro-analysis  was  practically  limited 
to  the  manner  in  which  certain  metals  could  be  deposited  quan- 
titatively from  their  pure  solutions  and  to  means  of  separating 
metals  from  solutions  containing  more  than  one  metal.  The  most 
favorable  experimental  conditions  were,  for  the  most  part,  discov- 
ered empirically,  as  is  always  the  case  during  the  first  stages  in 
the  development  of  a  new  branch  of  science.  It  is  worthy  of 
mention  that  the  development  of  the  theory  of  solutions  and 
the  improvement  of  practical  electro-analysis  took  place  almost 
simultaneously. 

The  fact  that  this  theory  of  solutions  soon  bore  fruit  in  the  field 
of  electro-analysis  is  not  to  be  wondered  at,  when  one  remembers 
that  the  theory  of  electrolytic  dissociation  is  one  of  the  main 
supports  of  electrolytic  reactions.  The  revolution  which  took 
place  in  electro-analytical  investigation  as  a  result  of  the  modern 
theoretical  conception  can  be.  best  illustrated  by  the  fact  that 
formerly,  in  searching  for  the  best  experimental  conditions,  chief 
stress  was  laid  upon  the  significance  of  the  quantities  of  electricity, 
as  determined  by  the  strength  of  the  current  and  the  current 
density,  but  gradually  the  significance  of  the  other  factor,  the 
voltage,  began  to  be  realized. 

However  undeniable  are  the  advantages  which  have  resulted 
from  the  theory  in  practical  electro-analysis,  it  must  not  be  for- 
gotten, on  the  other  hand,  that  a  powerful  impulse  toward  the 
development  of  the  theory  was  furnished  by  the  success  which 
characterized  analyses  made  in  this  way.  The  neatness  of  electro- 
analytical  methods,  the  accuracy  of  the  results,  and  the  rapidity 
of  the  reactions  soon  won  the  respect  of  both  scientific  and 
industrial  laboratories  and  it  acted  as  a  particular  stimulus 
upon  theoretical  investigation  to  realize  that  the  results  ob- 

*  Z.  Elektrochem.,  9,  760  (1903). 


84  QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 

tained  were  duly  appreciated  by  practical  men.  The  formula  on 
page  27, 

0.058  .      P 
E180=  —  log-, 

proposed  by  Nernst  in  1889,  which  permits  one  to  compute  the 
potential  difference  E  between  a  metal  and  a  solution  contain- 
ing its  ions,  from  the  electrolytic  solution  tension  P  and  the 
osmotic  pressure  p,  has  proved  of  great  practical  importance  for 
the  problems  of  galvanic  polarization  as  well  as  for  the  problems 
connected  with  the  galvanic  production  of  the  current.  The 
latter,  to  be  sure,  plays  a  subordinate  role  since  dynamos  have 
become  the  common  source  of  the  electric  current.  On  the  other 
hand,  most  of  the  problems  with  regard  to  the  rational  deposition 
of  metals  are  closely  related  to  this  Nernst  formula  and  its  signifi- 
cance for  electro-analysis  will  be  clear  after  reading  the  following 
discussion. 

When  a  metal  is  deposited  from  a  solution  by  means  of  an 
electric  current,  the  first  question  that  arises,  as  Danneel  correctly 
remarked,  is  this:  Under  what  conditions  is  the  metal  deposited? 
In  other  words,  what  application  of  energy  is  required  to  transform 
the  metal  from  its  ionic  into  its  atomic  condition,  so  that  it  will 
deposit  upon  the  cathode?  As  regards  the  quantity  of  electricity, 
we  need  not  pay  any  attention  to  this  for  the  present.  A  per- 
fectly analogous  question  to  the  above  would  be  this:  What 
temperature  is  requisite  for  the  coagulation  of  albumin?  We 
know  that  a  temperature  of  at  least  70°  is  necessary  to  coagulate 
an  albumin  solution  and  that  there  is  no  coagulation,  no  matter 
how  much  water  is  present,  provided  the  temperature  is  kept 
below  70°.  Just  as  in  this  case  it  does  not  make  any  difference 
how  much  heat  energy  is  present,  provided  the  temperature  is  not 
high  enough  for  coagulation,  so,  in  the  same  way,  no  matter  how 
much  electricity  is  conducted  through  the  solution,  there  will  be  no 
deposition  of  metal  unless  the  current  has  a  certain  voltage.  The 
Nernst  formula  tells  us  about  this  potential.  One  way  to  read 
the  formula  is  as  follows:  If  the  electrolytic  solution  tension  is 
equal  to  P  and  the  osmotic  pressure  equal  to  p  then  the  metal 
on  being  dipped  in  the  solution  will  have  the  voltage  E.  When 
read  this  way,  we  consider  E  as  a  function  of  the  two  other  values 
and  it  is  equally  accurate  to  read  the  formula  as  follows :  In  order 


THE   DEPOSITION  OF  METALS  85 

to  deposit  a  metal  from  a  solution  in  which  it  has  the  electrolytic 
solution  tension  P  and  the  osmotic  pressure  p,  an  electromotive 
force  of  at  least  E  volts  is  requisite. 

Thus,  in  order  to  compute  E,  we  must  know  the  values  P  and  p. 
There  are  well-known  methods  for  determining  the  osmotic  pres- 
sure p.  The  only  way  of  determining  the  electrolytic  solution  ten- 
sion is  to  measure  the  potential  E  experimentally  and  from  this, 
together  with  a  knowledge  of  p,  compute  the  value  P.  This  round- 
about method  is  necessary  because,  as  we  must  not  forget,  what 
we  term  electrolytic  solution  pressure  is  not  a  sensible  pressure 
which  can  be  measured.  The  conception  of  electrolytic  solution 
tension  is  merely  a  postulate  of  the  theory,  and  we  can  only  say 
that  metals  in  contact  with  a  solution  act  as  if  they  were  sending 
out  ions  with  a  certain  force.  In  other  words,  we  ascribe  to  the 
metals  the  power  to  send  ions  into  the  solution  and  this  tendency 
is  unchangeable  for  each  metal  at  a  constant  temperature  but 
varies  with  different  metals  and  at  different  temperatures.  Thus 
the  value  P  may  be  regarded  as  a  constant,  dependent  upon  the 
nature  of  the  metal. 

The  aim  should  be,  as  Danneel  stated,  to  ascertain  the  solution 
tensions  of  all  metals;  this  knowledge  will  place  us  in  a  position 
to  compute  the  various  voltages  required,  at  a  given  concentration 
of  the  ions,  and  we  shall  then  have  a  proper  scientific  basis  for 
separating  metals  from  one  another. 

Enough  has  been  said  to  indicate  the  importance  of  the  voltage 
measurements  described  on  page  36  et  seq.,  and  it  should  be 
emphasized  that  modern  investigation  makes  use  of  these  means 
most  thoroughly. 

Besides  the  question  concerning  the  requisite  conditions  for  the 
deposition  of  a  metal,  Danneel  enunciated  the  no  less  important 
questions:  " In  what  condition  do  the  metals  deposit?  What  are 
the  properties  of  the  deposits?  " 

The  importance  of  these  questions  has  been  pointed  out  on 
page  52.  For  analytical  purposes,  it  is  a  general  rule  that  the 
deposit,  aside  from  being  chemically  pure,  must  be  dense  and 
have  a  smooth  surface,  because  it  is  only  when  the  metal  is  in  such 
a  condition  that  it  can  be  washed  and  weighed  without  loss  and 
without  undergoing  change  by  oxidation.  The  conditions  which 
may  cause  an  uneven  deposition  were  studied  by  Danneel,  who 
took  as  an  example  the  deposition  of  silver  from  potassium-cyanide 


86  QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS 

solution,  which  requires  the  decomposition  of  the  complex  salt 
K[Ag(CN)2]. 

If  an  uneven  deposit  is  formed,  the  most  apparent  cause  is  that 
more  metal  had  been  deposited  at  the  same  time  on  some  parts  of 
the  electrode  than  on  others.  What  is  the  explanation  of  this 
behavior?  If  we  attempt  to  draw  a  picture  of  the  transport  of  the 
ions  through  the  electrolyte  until  they  are  discharged  at  the 
cathode,  we  find  that  the  current  lines,  i.e.,  the  paths  along  which 
the  ions  are  carried,  are  not  always  the  shortest  distances  between 
the  electrodes.  Thus  we  find  when  a  conical  platinum  electrode 
is  used,  with  no  openings  on  the  sides  (see  p.  54),  that  copper 
is  deposited  on  the  outside  of  the  cone  as  well  as  on  the  inside. 
On  the  other  hand,  we  must  assume  that  the  current  will  always 
seek  the  most  convenient  path;  thus  in  many  cases  a  scattering 
of  the  current  lines  is  observed.  If  we  take  a  corrugated  cathode, 
then  at  the  beginning  of  the  electrolysis,  when  the  ion  concen- 
tration is  the  same  at  all  parts  of  the  cathode  surface,  *the  most 
convenient  path  for  the  current  to  take  is  that  leading  to  the  ridges 
on  the  electrode,  and  the  current  lines  will  be  directed  toward  these 
high  places  and  there  the  first  deposit  of  metal  will  be  noticed. 
In  this  way  the  solution  in  the  vicinity  of  the  ridges  becomes 
robbed  of  its  metal  ions  and  the  result  of  this  is,  as  the  Nernst 
formula  indicates,  that  the  decomposition  potential  is  increased; 
this  is  because  a  lessening  of  the  ionic  concentration  causes  a  dimi- 
nution in  the  osmocic  pressure  p  and  if  this  value  is  diminished  the 
formula  shows  that  E  becomes  larger  (p.  84).  This  increase  in 
the  decomposition  potential  at  the  ridges  of  the  electrode  causes 
the  current  lines  to  be  directed  no  longer  toward  them  and  these 
lines  now  find  the  path  toward  the  indentations  of  the  electrode 
the  most  convenient  one.  Soon,  however,  the  latter  portions  of 
the  electrode  are  robbed  of  metal  ions  sufficiently  so  that  the 
current  lines  are  turned  away  from  them  and  are  again  directed 
toward  the  projections  on  the  surface  of  the  electrodes. 

Now,  if  we  follow  the  course  of  electro-analysis  still  farther,  we 
may  next  ask:  What  tends  to  prevent  the  impoverishment  of  the 
metal  ions  at  the  cathode?  In  the  first  place,  the  supply  of  metal 
ions  is  favored  by  the  migration  of  the  ions,  which  is  a  result  of 
the  action  of  the  current  (see  p.  13);  in  this  migration  the 
positively  charged  metal  ions  are  repelled  from  the  anode  and 
attracted  by  the  cathode.  There  is  here  a  marked  difference 


THE  DEPOSITION  OF  METALS  87 

noticed  according  to  whether  the  electrolyte  contains  a  simple  or 
a  complex  metal  salt.  In  the  solution  of  a  simple  metal  salt,  such 
as  silver  nitrate,  the  metal  ions  can  move  in  only  one  direction  and 
that  is  toward  the  cathode.  In  the  solution  of  a  complex  salt, 
on  the  other  hand,  the  electrolytic  dissociation  takes  place  in  such 
a  way  that  the  positively  charged  potassium  ions  are  attracted 
toward  the  cathode  while  the  negatively  charged  [Ag(CN)2p 
ions  move  toward  the  anode.  The  latter  ions  are,  to  a  very 
slight  extent,  dissociated  into  Ag  +  and  CN~  ions,  so  that  a 
limited  supply  of  silver  ions  is  present  at  the  cathode.  The  metal 
ions,  therefore,  will  be  supplied  much  more  slowly  in  complex 
electrolytes  than  in  a  simple  electrolyte. 

Although  the  metal  present  in  a  complex  anion  migrates  away 
from  the  cathode  it  is  clear  that  if  the  metal  is  present  in  a  complex 
cation,  as  in  the  ammoniacal  solution  of  a  silver  salt,  it  then  will 
migrate  in  the  opposite  direction.  Thus,  the  silver-ammonia 
cation  [Ag(NH3)2]+  migrates  toward  the  cathode. 

In  solutions  of  simple,  as  well  as  of  complex,  salts  the  supply  of 
metal  ions  is  also  supported  by  diffusion,  i.e.,  by  the  equalization 
of  the  metal  concentrations  in  the  impoverished  and  in  the  richer 
parts  of  the  solution.  Diffusion  will  be  chiefly  toward  those  parts 
of  the  solution  where  the  impoverishment  of  metal  ions  has  been 
the  greatest,  i.e.,  toward  the  ridges,  the  edges  and  corners  of  the 
cathode. 

It  was  pointed  out  on  page  63  how  this  diffusion  could  be  has- 
tened by  violent  stirring.  Attention  was  called  to  the  fact  that 
there  would  be  a  difference  in  the  effectiveness  of  the  stirring 
according  to  whether  the  natural  tendency  of  an  electrolyte  is 
to  furnish  metal  ions  slowly  or  quickly. 

It  is  easy  to  realize  that  the  rate  at  which  the  solution  is  im- 
poverished is  dependent,  to  a  high  degree,  upon  the  current 
density.  Danneel  sought  out  all  the  conditions  which  lead  to  an 
impoverishment  of  the  solution  and  all  those  which  had  the 
opposite  effect,  and,  after  contrasting  these,  he  attempted  to 
determine  the  effect  that  a  preponderance  of  one  or  the  other  of 
these  causes  would  have  upon  the  nature  of  the  deposit.  He  came 
to  the  following  conclusions :  If  the  current  density  is  so  low  that 
diffusion  has  time  to  prevent  any  serious  impoverishment  of  the 
solution  wherever  it  is  in  contact  with  the  electrode,  throughout 
its  entire  surface,  then  the  most  convenient  path  for  the  current 


88  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

lines  to  follow  is  to  the  elevated  portions  of  the  electrode,  and 
the  metal  deposit  develops  long,  well-formed  crystals,  such  as  are 
observed  in  a  silver  coulomb-meter  when  very  low  current  densities 
are  employed.  With  moderate  current  densities,  the  behavior 
of  the  solution  is  that  outlined  on  page  86,  and  the  deposit  takes 
place  alternately  upon  the  elevations  and  upon  the  depressions  of 
the  electrode  surface;  as  a  result  the  metal  is  uniformly  deposited 
over  the  entire  electrode.  If,  however,  the  current  density  is 
high  and  the  solution  is  impoverished  so  rapidly  that  diffusion  has 
no  chance  to  keep  up  the  supply  of  metal  ions,  then  the  behavior 
mentioned  on  page  86  is  encountered;  the  discharge  potential  of 
the  metal  increases,  because  of  the  diminution  in  the  osmotic 
pressure  of  its  ions,  until  it  becomes  equal  to  the  discharge  poten- 
tial of  hydrogen  and,  as  mentioned  on  page  53,  because  of  the 
simultaneous  discharge  of  hydrogen  ions  the  metal  is  deposited 
loosely  and  in  a  spongy  condition. 

The  foregoing  explanation  is  in  perfect  accord  with  the  fact 
mentioned  on  page  63,  that,  by  energetic  stirring,  the  evolution 
of  hydrogen  can  be  prevented  even  with  high  current  densities. 
H.  J.  S.  Sand*  pointed  this  out  in  1900.  This  explanation  also 
accounts  for  the  fact,  known  for  a  long  time,  that  certain  metals 
are  deposited  much  more  uniformly  from  the  solution  of  a  complex 
salt  than  from  one  of  a  simple  salt,  although  we  have  not  yet 
explained  why  the  complex  salts  are  better  suited  for  certain  metals 
than  for  others.  It  was  mentioned  above  that  silver  is  usually 
deposited  in  coarse  crystals  from  the  solution  of  a  simple  silver 
salt;  from  a  solution  of  the  complex  salt  K[Ag(CN)2],  on  the 
contrary,  the  silver  deposits  more  uniformly  and  we  can  explain 
this  by  the  slow  breaking  down  of  the  [Ag(CN)2]~  anion;  the  few 
silver  ions  present  in  the  solution  at  the  start  are  sent  toward  the 
elevations  on  the  surface  of  the  platinum  electrode,  then  the 
current  is  directed  toward  the  lower  portions  of  the  surface  where 
there  are  still  some  silver  ions,  and  during  this  time  the  anion 
[Ag(CN)2]~  is  decomposed  enough  to  restore  the  disturbed  equi- 
librium, and  the  process  is  repeated  over  and  over  again. 

The  conditions  mentioned  above,  under  which  the  metal  deposit 
is  loosened  by  or  accompanied  by  the  evolution  of  hydrogen,  will 
be  brought  out  more  clearly  by  the  following  logical  conclusions 
drawn  from  the  Nernst  formula.  They  show,  in  connection  with 

*Z.  phys.  Chem.,  35,  648  (1900). 


THE  DEPOSITION   OF   METALS  89 

the  theory  of  overvoltage  outlined  on  page  82:  1.  Why  it  is 
possible  to  precipitate  quantitatively  a  metal  from  a  solution  by 
electrolysis,  i.e.,  until  the  last  weighable  traces  are  removed,  in 
spite  of  the  increasing  impoverishment  of  the  metal  ions  and 
the  resulting  decrease  in  the  osmotic  pressure,  which  causes  the 
decomposition  potential  to  rise.  2.  Why  certain  metals,  although 
the  discharge  potential  of  their  ions  is  greater  than  that  of  hydro- 
gen, can  be  deposited  before  hydrogen  is  liberated. 

Let  us  first  examine  what  information  the  Nernst  formula  shows 
concerning  the  diminution  of  the  concentration  of  the  solution, 
as  this  is  something  which  must  take  place  in  every  electrolysis. 
Choosing  a  bivalent  metal,  for  simplicity,  then,  since  n  =  2,  the 
Nernst  formula  reads: 

E  =  0.029  log-- 

The  electromotive  force  E  has,  therefore,  a  definite  value  for  a 
given  osmotic  pressure  p,  i.e..  for  a  definite  concentration  of  metal 
ions. 

As  the  concentration  diminishes  during  the  progress  of  the  elec- 
trolysis eventually  the  value  of  p  sinks,  for  example,  to  one  tenth 
its  original  value.  Then  the  formula  becomes 

E!  =  0.029  log-^Q  =  0.029  log  10-  =  0.029  (log  10  +  log-), 


or  E!  =0.029  +  0.029  log  -, 

Thus,  when  the  dilution  is  increased  tenfold,  the  electromotive 
force  E  is  only  increased  0.029  volt. 

Similarly,  when  the  concentration  of  the  solution  has  been 
reduced  until  p  is  only  TJT  of  its  original  value,  then 


or 


E2  =  0.029  Aog  100  +  log-Y 


E2  =  2X0.029  +  0.029  log-- 


This  computation  shows,  therefore,  that  for  every  time  the 
solution  is  diluted  tenfold,  the  value  of  E  is  increased  0.029  volt,* 

*  If  the  metal  in  consideration  is  univalent,  this  value  becomes  0.058 
volt,  if  trivalent  0.019  volt,  and  if  quadrivalent  0.015  volt.     In  other  words, 


90  QUANTITATIVE  ANALYSIS  BY   ELECTROLYSIS 

and  thus  if  the  concentration  were  diminished  until  the  osmotic 

/Y\ 

pressure  of  the  solution  became  -^,  the  value  of  E  would  be  in- 
creased only  6  X  0.029  =  0.174  volt,  or  not  quite  0.2  volt. 

When  the  concentration  has  been  reduced  to  one-millionth  of  its 
original  value,  the  quantity  of  metal  remaining  cannot  be  detected, 
in  most  cases,  by  the  ordinary  reagents  and  the  deposition  may 
be  regarded  as  complete.  Mathematically,  it  will  be  impossible 
ever  to  reach  the  true  zero  concentration.  The  significance 
of  the  increase  in  potential  of  nearly  0.2  volt  will  be  shown 
at  once. 

If  we  examine  the  Nernst  formula  to  determine  under  what 
conditions  a  metal  can  be  deposited,  we  shall  find  that  it  shows  us 
the  conditions  under  which  the  potential  of  the  metal  remains 
smaller  than  that  of  hydrogen,  i.e.,  when 

0.029 log-  <  0.058  log  —  • 

P  and  p  refer  to  the  metal,  PH  and  p//  to  hydrogen. 

In  the  course  of  the"  analysis  the  above  inequality,  which  must 
persist  for  the  desired  purpose,  changes;  the  expression  on  the 
left  becomes  larger,  because  p  grows  smaller,  and  the  expression  on 
the  right  becomes  smaller,  because  p,  which  is  proportional  to  the 
concentration  of  the  hydrogen  ions,  is  usually  increased  by  the 
formation  of  acid. 

There  is,  therefore,  a  tendency  for  the  two  sides  of  the  above 
inequality  to  become  equal  to  one  another,  or,  in  the  most  unfavor- 
able case,  for  the  potential  of  the  hydrogen  to  become  greater  than 
that  of  the  metal. 

The  following  table  gives  the  discharge  potentials  in  volts  for 
six  metals  from  normal  solutions  as  determined  for  moderate 
current  densities  at  the  cathode.* 

where  the  change  in  valence  is  one,  changing  the  concentration  tenfold 
changes  the  electromotive  force  required  to  discharge  it  0.058  volt  at  18°; 
if  the  valence  change  is  n  a  corresponding  change  of  concentration  changes 

C\  P\Q 

the  electromotive  force  —  -  volt. 
n 

*  Coffetti  and  Foerster,  Ber.,  38, 2934,  and  Z.  angew.  Chem.,  19, 1842  (1906). 
The  values  here  given  are  in  round  numbers  and  those  for  Cd  have  been 
obtained  to  some  extent  by  interpolation.  For  the  details  of  making  the 
measurements,  the  original  paper  should  be  consulted. 


THE   DEPOSITION   OF  METALS 


91 


Current  density 

in  amperes  per 

Zn. 

Fe. 

Ni. 

Co. 

Cd. 

Cu. 

sq.  cm. 

0 

+0.79 

+0.66 

+0.60 

+0.52 

+0.44 

-0.31 

0.0023 

+0.84 

+0.71 

+0.63 

+0.56 

+0.49 

-0.27 

0.0046 

+0.85 

+0.73 

+0.65 

+0.58 

+0.50 

-0.26 

0.0091 

+0.88 

+0.75 

+0.66 

+0.59 

-0.24 

The  values  in  the  above  table  represent  the  difference  between 
the  discharge  potentials  of  the  metals  and  that  of  hydrogen  from 
a  normal  solution  of  hydrogen  ions.  Thus,  if  the  last  value  given 
for  copper  is  inserted  in  the  above  inequality,  it  becomes 

-  0.24  <  0 

and  the  expression  shows  that  it  is  possible  to  deposit  copper  from 
an  acid  solution.  The  inequality  remains  in  the  same  sense  if  we 
assume  the  maximum  value  applicable  to  the  extremely  dilute 
copper  solution,  at  which  the  discharge  potential  of  the  copper  will 
be  not  more  than  0.2  amperes  in  addition  to  its  previous  value,  for 

-  0.24  +  0.2  <  0 

In  other  words,  copper  can  be  deposited  completely,  or  at  least 
to  within  the  limits  that  can  be  detected  qualitatively,  from  an 
acid  solution  of  a  simple  salt. 

The  more  noble  metals,  mercury,  silver,  etc.,  behave  like  copper 
in  this  respect,  because  their  position  in  the  potential  table  is  to 
the  right  of  copper. 

Other  metals,  like  cadmium,  have  discharge  potentials  which  are 
more  positive  than  that  of  hydrogen  and  they  should  not,  in 
accordance  with  this  view,  be  deposited  before  hydrogen  is  liber- 
ated, for  the  reversed  inequality  expression  becomes 

0.029  log ->  0.058  log  — . 
P  PH 

As  a  matter  of  fact,  cadmium  can  be  deposited  from  fairly  acid 
solutions  and  the  reason  for  this  is  to  be  sought  in  the  overvoltage, 
mentioned  on  page  82,  which  hydrogen  shows  to  these  metals. 
The  inequality  expression  which  expresses  the  condition  for  the 
possibility  of  the  deposition  of  these  metals,  takes  the  following 
form: 

0.029  log  -  <  0.058  log  —  +  r/, 
P  PH 


92 


QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS 


in  which  77  represents  the  overpotential  in  volts   for  hydrogen 
toward  the  metal  in  question. 

Foerster*  took  values  determined  by  J.  Tafelf  and  arranged 
them  in  the  following  table : 


Current  density 
in  amperes  per 
sq.  cm. 

Overpotential  of  hydrogen  in  volts,  on 

Hg.* 

Sn. 

Cu. 

Ni. 

Pt,  platinized. 

0.01 
0.05 
0.10 

1.18 
1.26 
1.30 

0.98 
1.11 
1.16 

0.57 
0.70 
0.79 

0.56 
0.68 
0.74 

0.05 
0.06 
0.08 

If  these  values  are  placed  on  the  right-hand  side  of  the  above 
inequality  expression,  the  possibility  of  the  metal  being  deposited 
before  hydrogen  will  be  shown. 

All  the  above  explanation  has  been  with  reference  to  solutions 
of  simple  salts,  and  especially  the  sulphates. 

It  has  been  pointed  out  that,  the  deposition  from  complex  salts 
takes  place  less  readily  than  from  simple  salts.  With  reference  to 
this  fact,  Foerster  §  has  collected  the  following  values,  which  show 
that  the  deposition  potentials  of  zinc,  copper  and  cadmium  in 
alkali-cyanide  solution  lie  much  higher  than  the  corresponding 
values  in  sulphate  solutions.  The  figures  given  in  the  last  three 
columns  of  the  table  hold  true  for  solutions  containing  ^  mole 
of  the  metal  cyanide  in  question,  and  this  is  designated  by  the 
general  symbol  M(CN)Z,  in  the  presence  of  T^  or  TV  and  \%  moles 
of  potassium  cyanide.  The  first  column  gives,  for  comparison, 
the  values  in  a  normal  solution  of  the  sulphate. 


M. 

\  mole  MSO4 
in  1  liter. 

T^moleMCCN)* 
+&  mole  KCN 
in  1  liter. 

A  mole  M  (CN)« 
+^5  mole  KCN 
in  1  liter. 

AmoleM(CN)* 
+1  mole  KCN 
in  1  liter. 

Zn       

volt 
+0.79 

volt 

+1.03 

volt 
+  1.18 

volt 
+  1.23 

Cd 

+0.44 

+0.71 

+0.87 

+0.90 

Cu   

—0.31 

+0.61 

+0  96 

+1  17 

From  this  table  it  is  clear,  (1)  that  the  potential  in  a  potassium- 
cyanide  solution  is  considerably  higher  than  in  a  sulphate  solution; 

*  Z.  angew.  Chem.,  19,  1843  (1906).     f  Z.  physikal.  Chem.,  50,  641  (1905). 
J  The  values  for  lead,  cadmium  and  zinc  are  close  to  those  for  mercury. 
§  Z.  angew.  Chem.,  19,  1846  (1906). 


INFLUENCE  OF  TEMPERATURE  ON  SEPARATION         93 

(2)  that  the  potential  increases  as  the  potassium-cyanide  content 
is  raised;  (3)  that  the  potential  of  copper  increases  relatively 
faster  than  that  of  the  other  two  metals  and,  under  the  experimental 
conditions  of  the  fourth  column,  it  is  even  greater  than  that  of 
cadmium.  Consequently,  in  a  solution  containing  considerable 
potassium  cyanide,  cadmium  will  be  deposited  before  copper, 
whereas  in  a  dilute  sulphuric-acid  solution  the  reverse  is  true. 
From  the  closeness  of  the  values  given  for  copper  and  zinc  in  the 
fourth  column,  it  is  clear  why  these  metals  can  be  deposited 
simultaneously  in  the  form  of  brass  from  a  potassium-cyanide 
solution,  which  is  altogether  impossible  in  a  sulphuric-acid  solution 
owing  to  the  difference  between  the  discharge  potentials  of  these 
metals  in  acid  solution. 

The  influence  of  heat  upon  the  deposition  and  separation  of 
metals  in  simple  and  complex  electrolytes  will  next  be  shown,  the 
data  being  taken  from  an  article  by  F.  Foerster.* 

Influence  of  Temperature  on  the  Separation  of  Metals  in  Com- 
plex Electrolytes. 

If,  during  the  electrolytic  deposition  of  a  metal,  the  potential 
at  the  cathode  is  measured  by  means  of  an  auxiliary  electrode 

eti   a  e  d  b  c 


;  0.5 


0.4 


0.3 


0.2 


0.1 


Ag 


18° 


3      0.3         0.4          0.5 


Ci 


(18° 


0.7          0.8          0.9          1.0          1.1         1.2   Yolt 
Discharge  Potential 

FIG.  45. 


(cf.  p.  40),  or,  in  other  words,  if  the  discharge  potential  is  meas- 
ured at  varying  current  densities  but  at  a  uniform  temperature  of 


Z.  Elektrochem.,  13,  561  (1907). 


94  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

the  electrolyte,  e.g.,  at  18°,  and  if  the  corresponding  values  for 
current  density  and  potential  are  plotted,  with  the  former  as 
ordinates  and  the  latter  as  abscissas,  it  will  be  shown  by  the  rapid 
rise  of  the  curve  that  the  cathode  potential  is  increased  but 
slightly  as  the  current  density  is  raised.  A  curve  obtained  in 
this  way  is  similar  to  that  of  a  in  Fig.  45.  This  is  generally 
true,  however,  only  for  simple  electrolytes,  such  as,  for  example, 
the  sulphate  solutions  of  copper,  cadmium  and  zinc. 

If  the  same  measurements  are  carried  out  for  a  given  metal  at  a 
higher  temperature,  e.g.,  50°,  the  curve  shows  a  steep  ascent  as 
before,  but  there  is  the  difference  that  the  new  curve  lies  to  the 
left  of  the  one  plotted  for  the  lower  temperature  because  the  dis- 
charge potentials  at  high  temperatures  are  lower  than  those  at 
low  temperatures.  This  diminution  of  the  potential  value  depends 
upon  the  decrease  of  the  resistance  of  the  electrolyte  caused  by 
heating  it.  Aside  from  this  lessening  of  the  resistance,  which 
naturally  corresponds  to  a  better  current  yield,  the  raising  of  the 
temperature,  as  the  author  was  the  first  to  point  out,  also  results 
in  an  improvement  in  the  nature  of  the  deposit  obtained;  it  is 
denser  and  adheres  more  firmly  to  the  cathode. 

If  corresponding  measurements  are  made  in  complex  electrolytes, 
e.g.,  in  a  potassium-cyanide  solution,  the  current  density  vs.  poten- 
tial curves  will  show  that  the  metals  behave  differently  with  respect 
to  the  increase  of  discharge  potential  with  rise  of  current  density. 

In  Fig.  45,  the  curve  a  represents  the  deposition  of  silver  from 
a  potassium-cyanide  solution  at  18°;  it  shows  that,  similar  to  the 
deposition  from  a  solution  of  a  simple  salt,  the  discharge  potential 
increases  but  slightly  with  increased  current  density.  The  curve 
ai,  lying  to  the  left  of  a,  represents  the  deposition  of  silver  from  a 
potassium-cyanide  solution  at  60°.  These  two  curves  show  that 
the  behavior  of  silver  in  potassium-cyanide  solution  is  similar,  in 
this  respect,  to  the  behavior  of  silver  in  a  simple  electrolyte. 

The  same  is  true  of  cadmium,  of  which  only  the  curve  b  at  18° 
is  drawn. 

Copper,  on  the  other  hand,  behaves  quite  differently  in  an 
alkali-cyanide  solution.  The  curve  c,  which  represents  copper  in 
a  potassium-cyanide  solution  at  18°,  shows  that  the  discharge 
potential  increases  considerably  at  this  temperature  with  increas- 
ing current  density.  The  curves  d  and  e,  for  35°  and  75°  respec- 
tively, show  that  the  behavior  of  copper  at  higher  temperatures 


INFLUENCE  OF  TEMPERATURE  ON   SEPARATION         95 

corresponds  more  nearly  to  its  behavior  in  the  solution  of  a  simple 
electrolyte.  Thus  the  curve  e,  being  so  nearly  a  vertical  line, 
makes  it  clear  that  the  discharge  potential  varies  but  slightly  with 
increasing  current  density. 

Zinc,  for  which  only  the  curve  /  at  18°  is  drawn,  behaves  like 
copper. 

Now,  if  we  study  the  curve  c  more  closely,  we  shall  find  that,  in 
the  deposition  of  copper  from  a  potassium-cyanide  solution  at 
ordinary  temperature,  the  discharge  potential  for  copper  increases 
considerably  as  the  current  density  is  raised,  or,  conversely,  if  it  is 
desired  to  deposit  copper  with  high  current  densities,  and  thus 
jnore  rapidly,  it  requires  a  much  higher  voltage.     A  comparison 
of  the  curve  c  with  the  curve  a,  for  silver  at  18°,  and  with  ait  for 
silver  at  60°,  shows  that  the  relations  are  much  more  favorable  for 
silver  inasmuch  as  the  potential  for  this  metal  in  cyanide  solutions 
whether  at  18°  or  at  60°,  increases  very  slightly  with  increasing 
current  density;    in  other  words,  a  slight  increase  in  the  voltage 
of  the  current  results  in  a  marked  increase  in  the  current  density 
and  a  much   more   rapid   deposition  of  the  metal.     Foerster* 
explains  the  behavior  of  copper  and  zinc  by  assuming  that  there  is 
a  " reaction  resistance"  to  overcome  in  the  case  of  the  cyanide 
solutions  of  these  metals.     This  resistance,  of  which  the  nature 
is  still  unknown,  is  lessened  by  raising  the  temperature,  as  the 
curve  e  for  copper  at  75°  clearly  shows.     The  fact  that  such  a 
reaction  resistance  is  not  shown  in  the  solution  of  silver  in  alkali 
cyanide  is  an  argument  against  the  assumption  that  such  a  resist- 
ance is,  in  general,  found  in  complex  electrolytes  and  that  the 
resistance  can  be  explained  by  the  difficulty  in  decomposing  the 
complex  that  contains  the  metal.     However,  it  must  be  remem- 
bered that  there  are  gradations  in  the  complexity  of  such  solutions, 
and  this  is  true  not  only  in  the  complexes  of  different  metals,  as, 
for  example,  between  the. copper-cyanide  ion  and  the  silver-cyanide 
ion,  but  also  in  the  complex  of  one  and  the  same  metal  at  different 
temperatures  and  concentrations  of  the  solution.     If  the  degree 
of  complexity  is  judged  by  the  anomalous  reactions  which  the 
solutions  show,  then  the  argenticyanide  ion  [Ag(CN)2]~  must  be 
regarded  as  less  complex  than  the  cuprocyanide  ion  [Cu2(CN)]|  -; 
the  solution  of  silver  cyanide  in  potassium  cyanide  gives  a  pre- 
cipitate when  treated  with  hydrogen  sulphide  while  this  is  not 

*  Z.  Elektrochem.,  13,  561  (1907). 


96  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

the  case  with  a  potassium-cyanide  solution  containing  dissolved 
copper.  Moreover,  the  complexity  of  the  potassium-cuprocyanide 
solution  becomes  increased  as  the  potassium-cyanide  content  is 
raised;  thus  F.  P.  Treadwell  and  v.  Girsewald*  have  found  that 
complete  complexity,  i.e.,  the  failure  of  the  usual  reactions  for 
detecting  copper,  especially  the  hydrogen-sulphide  test,  is  only 
brought  about  when  the  solution  contains  more  than  enough 
potassium  cyanide  to  form  the  salt  K6[Cu2(CN)8]  (cf.  p.  48). 
The  two  facts  discovered  by  A.  B  runner  f  are  in  accord  with  this. 
Brunner  found  that  by  increasing  the  amount  of  potassium  cyanide 
added  to  the  solution  he  could  prevent  the  electrolytic  deposi- 
tion of  copper  and  that  at  a  higher  temperature,  as  the  curve  e 
shows,  the  deposition  took  place  normally.  In  the  case  of  copper, 
therefore,  the  influence  attributed  to  reaction  resistance,  which  in- 
fluences the  velocity  of  the  metal  deposition  with  high  potassium- 
cyanide  content  and  low  temperatures,  can  be  explained  by  the 
highly  complex  nature  of  the  solution. 

Whatever  the  truth  of  the  matter  may  be,  the  experiments  of 
Foerster  and  his  co-workers  have  served  to  explain  a  number  of 
important  facts  already  known  concerning  electrolytic  deposition. 
Thus,  for  example,  cadmium  can  be  deposited  before  the  copper 
in  a  potassium-cyanide  solution  with  a  current  of  2.6  volts,  pro- 
vided sufficient  potassium  cyanide  is  present.  The  possibility  of 
this  separation  cannot  be  traced  to  the  difference  in  potential 
between  the  two  metals,  for  it  is  only  about  0.2  volt  in  such  a 
potassium-cyanide  solution  and  this  is  not  sufficient  for  a  satis- 
factory separation.  The  separation  is  based  rather  upon  the 
different  reaction  velocities  with  which  the  metals  are  deposited 
under  the  given  conditions. 

The  curve  e  shows  that  this  reaction  velocity  is  much  greater 
for  copper  at  75°  and  it  then  is  very  near  to  the  reaction  velocity 
of  cadmium,  and  since  the  discharge  potentials  of  the  two  metals 
are  near  one  another  at  this  temperature,  it  is  clear  that  it  is 
impossible  to  effect  a  satisfactory  separation  at  high  temperatures. 

The  opposite  case,  where  a  separation  can  take  place  at  a  high 
temperature  although  impossible  at  the  ordinary  temperature,  will 
be  discussed  in  the  separation  of  nickel  from  zinc. 

t  Z.  anorg.  Chem.,  38,  92  (1904). 
j  Z.  Elektrochem.,  13,  562  (1907). 


NON-ELECTROLYTIC  METHODS  OF  ANALYSIS  97 


Non-electrolytic  Methods  of  Electrochemical  Analysis. 

This  book,  according  to  its  title,  embraces  methods  of  elec- 
trolytic analysis  in  which  the  metal  is,  as  a  rule,  deposited  upon 
the  cathode  and  weighed  in  the  metallic  condition.  There  are, 
however,  three  other  methods  of  quantitative  analysis  which  are 
associated  with  the  electrochemistry  of  aqueous  solutions.  These 
are  (1)  potential  measurements  which  serve  to  determine  the 
concentration  of  ions  in  very  dilute  solutions;  (2)  conductivity 
measurements  which  are  often  convenient  for  determining  the 
concentration  of  solutions;  and  (3)  electrometric  titrations  in 
which  the  end-point  of  a  reaction  is  determined  by  a  sudden 
change  in  the  decomposition  potential  at  the  cathodc  The 
theory  of  these  processes  is  so  closely  related  to  that  of  ordinary 
electrolytic  work  that  it  seems  desirable  to  discuss  the  prin- 
ciples briefly  at  this  point  and  to  include  a  few  such  methods  in 
the  following  sections  of  the  book. 

We  have  seen  (page  28),  that  the  electromotive  force  developed 
at  18°  by  contact  of  a  metal  with  its  ions  may  be  expressed  mathe- 
matically by  the  Nernst  formula: 

0.058 ,      P 

#18°  =  ~       ~  log  —  VOltS. 

n  p 

Since  the  osmotic  pressure  is  proportional  to  the  concentration 
of  the  dissolved  ions,  it  is  mathematically  correct  to  substitute 
the  ionic  concentration  of  the  solution,  c,  for  the  osmotic  pres- 
sure, p,  and  to  replace  the  solution  pressure,  P,  by  the  ionic 
concentration,  C,  which  prevails  in  the  solution  when  E  =  0. 
The  Nernst  formula  then  becomes 

0.058 ,      C      ,, 

#18°  =    -       -  log  —  VOltS. 

n  c 

Since  the  logarithm  of  10  is  1  and  of  0.1  is  —  1,  the  value 

Q 

of  log  —  ( =  log  C  —  log  c)  is  decreased  one  unit  if  the  ionic  con- 
c 

centration,  c,  is  increased  tenfold  and  the  value  is  increased 
one  unit  if  c  is  decreased  to  one-tenth  its  former  value.  The 

0  58 
value  of  E,  therefore,  is  decreased  -    -  volt  if  the  concentration 


98  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

of  the  solution  is  increased  tenfold  and  increased  by  the  same 
value  if  the  solution  is  diluted  tenfold. 

If  a  silver  electrode,  for  example,  is  placed  in  a  0.1  normal 
solution  of  silver  nitrate,  and  another  silver  electrode  is  placed 
in  a  0.01  normal  solution  of  silver  nitrate,  then  on  joining  the 
electrodes  by  means  of  a  wire  and  placing  the  two  solutions  in 
contact  with  one  another,  a  current  will  flow  through  the  wire 
from  the  concentrated  solution  to  the  dilute  one  and  its  electro- 
motive force  will  be  0.058  volt;  the  silver  will  dissolve  in  the 
dilute  solution  and  will  be  deposited  from  the  concentrated  solution. 

This  principle  may  be  applied  to  the  determination  of  the 
solubility  of  difficultly  soluble  substances.  A  convenient  way 
of  doing  this  is  to  determine  the  decomposition  potential  at  the 
cathode,  with  the  aid  of  a  hydrogen  electrode,  or  a  normal  cal- 
omel electrode,  using  a  solution  of  known  ionic  composition 
at  one  electrode  and  a  saturated  solution  of  the  difficultly  soluble 
substance  at  the  other.  Applying  the  Nernst  formula  to  the 
two  solutions  of  concentrations  c\  and  02,  and  subtracting  one 
from  the  other  to  get  the  difference  in  potential,  we  have 

0.058,      C      0.058,      C 

EIS°  = log  -  -  -     -  log  - 

n  02         n  c\ 

.058  /i      >-*      *  i      ^  i  i        \     0.058 ,      Ci 

= (logC  -  log  c2  -  logC  +  logci)  = log  -. 

n  n  €2 

To  illustrate,  if  the  measured  value  of  E  is  0.216  in  a  cell  of 
which  the  cathode  is  silver  against  a  tenth-normal  solution  of 
silver  ions  and  the  anode  is  silver  against  a  saturated  solution  of 
a  slightly  soluble  silver  salt,  then 

0.1      0.216 

log^=oo58  =  3-73; 

—  =  5.37  X  10+3; 

C2 

c2  =  1.86  X  10  -5. 

The  solubility  of  the  silver  halides  may  be  determined  in  this 
way. 

The  application  of  the  conductivity  principle  to  the  concen- 
tration of  solutions  involves  the  same  principle  as  when  any 
other  physical  property,  such  as  specific  gravity,  is  used  for  the 


NON-ELECTROLYTIC  METHODS  OF  ANALYSIS  99 

purpose.  The  specific  gravity  of  all  mixtures  of  water  and  alcohol 
is  known;  by  determining  the  specific  gravity  of  a  mixture  of 
alcohol  and  water,  therefore,  it  is  easy  to  find  out  the  percentage 
of  alcohol  present  by  consulting  tables  that  have  been  prepared. 
In  the  same  way,  if  the  conductivity  of  solutions  of  any  elec- 
trolyte is  known  for  various  dilutions  it  is  possible  to  tell  what 
the  concentration  of  a  solution  is  by  measuring  the  conductivity. 
Thus  the  small  quantity  of  mineral  salt  present  as  impurity  in 
a  sugar  solution  or  in  a  mineral  water  can  be  determined  fairly 
well  by  measuring  the  conductivity. 

The  principle  involved  in  electrometric  titrations  is  similar 
to  that  of  determining  solubility  by  measuring  the  cathode 
potential.  The  quantitative  methods  of  acidimetry  and  alkalim- 
etry consist  in  measuring  the  concentration  of  the  hydrogen 
ion.  Ordinarily  an  indicator,  such  as  methyl  orange,  methyl  red, 
or  phenolphthalein,  is  used  which  changes  color  at  a  certain 
definite  concentration  of  hydrogen  ions. 

Kohlrausch  and  Heydweiller*  have  determined  the  conductivity 
of  very  pure  water.  Assuming  that  its  conductivity  is  due  to 
the  presence  of  an  equal  number  of  hydrogen  and  hydroxyl  ions, 
the  concentration  of  each  was  found  to  be  10  ~7  in  moles  per 
liter.  If  we  designate  the  concentration  of  hydrogen  ions  by 
[H+],  that  of  hydroxyl  by  [OH~]  then  the  mass  action  law  applied 
to  the  ionization  of  water  reads 

[H+]  X  [OH-] 

— ru~7vi =  a  constant. 

[H2O] 

Since  the  total  volume  of  the  water  is  not  influenced  appreciably 
by  the  ionization,  and  its  value  is  very  large  in  comparison  to 
the  concentration  of  the  hydrogen  and  hydroxyl  ions,  we  may  say 
that  the  equilibrium  between  H+  and  OH~  ions  can  be  expressed 
in  all  cases  by  the  equation : 

[H+]  X  [OH"]  =  1(T14. 

In  a  0.001-normal  acid  solution  the  concentration  of  the  hydrogen 
ions  is  10~3  but  in  a  0.001-normal  caustic  alkali  solution  the 
concentration  of  the  hydrogen  ions  comes  entirely  from  the 
water  and  since  the  hydroxyl  concentration  is  0.001,  it  follows 
that  the  hydrogen  ion  ^concentration  is  10"  u.  In  other  words, 
*  Wied.  Ann.,  53,  209  (1894). 


100  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

as  the  solution  changes  from  the  acid  side  to  the  alkaline  side 
during  the  addition  of  alkali,  a  sudden  change  takes  place  in 
the  concentration  of  the  hydrogen  ion.  If  the  cathode  potential 
is  compared  with  that  of  the  hydrogen  electrode,  or  of  the  normal 
calomel  electrode,  and  a  galvanometer  is  placed  in  the  circuit, 
the  needle  of  the  galvanometer  will  be  deflected  suddenly  when 
the  acid  is  just  neutralized.  Bottger  *  has  applied  this  prin- 
ciple to  the  titration  of  a  number  of  acids  and  bases.  J.  H.  Hilde- 
brand  f  has  shown  how  the  experimental  technique  can  be  sim- 
plified. 

We  have  seen  that  all  chemical  changes  that  take  place  at  the 
cathode  are  reductions  and  when  we  measure  the  cathode  de- 
composition potential  in  a  solution  of  hydrochloric  acid  we  are 
really  measuring  the  electromotive  force  necessary  to  reduce 
the  hydrogen  from  the  positively  charged  to  electrically  neutral 
condition.  Any  other  reaction  of  oxidation  and  reduction  may 
be  studied  in  the  same  way.  Thus  Crotogino  |  has  determined 
the  end-point  in  oxidation  and  reduction  reactions  with  the  use 
of  a  platinum  electrode  and  galvanometer.  Ostwald,  Luther 
and  Drucker,§  Hildebrand  ]f  and  Forbes  and  Bartler  **  have 
discussed  in  particular  the  titration  of  ferrous  salts  with  potas- 
sium dichromate.  These  electrometric  titrations  are  particularly 
useful  in  solutions  which  are  highly  colored  so  that  ordinary 
indicators  are  not  helpful. 

*Z.  phys.  Chem.,  24,  253  (1897).    See  also  van  Suchtelen  and  Itano. 
J.  Am.  Chem.  Soc.,  37,  1793  (1915). 
t  J.  Am.  Chem.  Soc.,  35,  845  (1913). 
t  Z.  anorg.  Chem.,  24,  225  (1900). 
§  Physikal-chemische  Messungen,  p.  454. 
K  J.  Am.  Chem.  Soc.,  35,  869  (1913). 
**  J.  Am.  Chem.  Soc.,  35,  1527  (1913), 


HISTORICAL. 

Like  every  new  branch  of  science,  the  development  of  electro- 
chemical analysis  was  at  first  almost  wholly  empirical.  The  most 
suitable  conditions  for  the  quantitative  separation  of  metals  by 
eleptricity  were  determined  from  a  great  number  of  experiments, 
conducted  with  diligence  and  perseverance,  while  the  nature  of 
the  reactions  involved  was  not  always  clearly  understood.  The 
relatively  recent  development  of  electrochemistry  has  served  to 
throw  much  light  on  the  theory  of  quantitative  electrolysis,  and 
the  importance  and  significance  of  the  electrical  factors  and  other 
conditions  are  now  much  more  clearly  understood. 

The  first  attempts  at  the  electrolytic  determination  of  the 
metals  were  entirely  qualitative  in  character.  Shortly  after  the 
discovery,  by  Nicholson  and  Carlisle  (1800),  of  the  decomposition 
of  water  by  the  electric  current,  Cruikshank  (1801),  having  ob- 
served the  separation  of  metallic  copper,  suggested  that  the 
galvanic  current  might  be  used  for  the  qualitative  determination 
of  other  metals.  This  suggestion  awakened  but  little  interest. 
In  1812  Fischer  employed  an  electrolytic  method  for  identifying 
arsenic  in  animal  fluids,  and  later,  in  1840,  Cozzi  used  a  similar 
method  for  the  detection  of  metals  in  general  in  such  solutions. 

The  discovery  of  galvanoplasty,  a  most  important  technical 
process  closely  allied  to  electrochemical  analysis,  dates  from  1839 
and  was  made  by  Jacobi. 

Gaultier  de  Claubry,  in  1850,  recommended  the  use  of  the 
electric  current  for  detecting  poisonous  metals  in  mixtures  con- 
taining organic  substances,  and  in  1860  Charles  L.  Bloxam  con- 
tinued this  work  and  devised  numerous  methods  by  which  he 
attempted  to  make  the  identification  of  arsenic  and  antimony 
possible  in  the  presence  of  other  metals.  In  this  work  he  was 
assisted  somewhat  by  the  directions  for  the  separation  of  metals 
from  mixtures  published  by  Morton  in  1851. 

Becquerel  observed,  as  early  as  1830,  that  lead  and  manganese 
often  separated  in  the  form  of  oxides  on  the  anode,  a  property 
which  permitted  these  metals  to  be  readily  separated  from  others 

101 


102  <^;£NTi£AT^Y      ANALYSIS  BY  ELECTROLYSIS 

which  deposit  on  the  cathode.  Investigations  chiefly  on  the 
qualitative  decomposition  of  inorganic  salts  of  the  metals  were 
also  carried  out  by  Despretz  (1857),  Nickles  (1862),  and  Wohler 
(1868).  The  work  of  A.  C.  and  E.  Becquerel  (1862)  on  the  elec- 
trolytic reduction  of  the  metals  was  likewise  of  an  entirely  qual- 
itative character. 

It  can  be  readily  understood  that  with  such  abundant  data  at 
hand  the  development  of  quantitative  electrolysis  could  now  take 
place  quite  rapidly. 

The  field  of  quantitative  investigation  was  first  opened  by 
W.  Gibbs  (1864),  who  carried  out  an  investigation  on  the  elec- 
trolytic determination  of  copper  and  nickel,  which  included  a 
description  of  the  methods  for  the  determination  of  silver  and 
bismuth  in  the  form  of  metals,  as  well  as  of  lead  and  manganese 
in  the  form  of  peroxides.  He  also  published  studies  on  the  separa- 
tion of  zinc,  nickel  and  cobalt.  The  possibility  of  the  quantitative 
determination  of  copper  was  confirmed  by  Luckow  (1865),  who 
had  worked  at  it  for  a  number  of  years.  The  quantitative  elec- 
trolytic determination  of  metals  was  entitled  by  him  "  electro- 
metal-analysis."  This  author  published  at  the  same  time  a  series 
of  directions  for  the  method  of  using  the  current  for  analytical 
work,  and  by  these  precise  instructions  laid  the  foundation  for 
many  later  researches. 

The  attention  of  investigators  was  first  turned  chiefly  toward 
the  chemical  reactions  taking  place  in  th'e  electrolytic  cell  and  less 
weight  was  placed  upon  the  source  of  the  current  and  the  physical 
condition  of  the  experiment.  The  metal  salts  most  suitable  for 
electrolysis,  the  best  solvents  and  the  proper  substances  to  be 
added  to  the  solutions  were  investigated  and  determined.  Thus 
Wrightson  (1876)  called  attention  to  the  fact  that  the  accuracy  of 
copper  determinations  was  influenced  by  the  presence  of  other 
metals  and  ascertained  the  limits  under  which  copper  could  be 
accurately  determined  in  the  presence  of  antimony.  The  results 
obtained  in  the  electrolytic  determination  of  cadmium,  zinc  and 
other  metals  were  not  yet  satisfactory. 

Simultaneously  with  the  announcement  of  the  electrolytic  deter- 
mination of  gallium  in  alkaline  solutions  by  Lecoq  de  Boisbaudran 
(1877)  came  the  announcement  by  Parodi  and  Mascazzini  that 
zinc  could  be  determined  in  a  solution  of  its  sulphate  to  which 
an  excess  of  ammonium  acetate  had  been  added,  and  that  metallic 


HISTORICAL  103 

lead  could  be  quantitatively  precipitated  from  an  alkaline  tartrate 
solution  containing  an  alkali  acetate. 

We  are  indebted  to  Riche  (1878)  for  the  first  accurate  directions 
for  the  determination  of  manganese.  He  observed  that  this 
element  may  be  completely  separated  at  the  positive  pole  in  the 
form  of  an  oxide  from  solutions  of  the  nitrate.  This  property 
permits  the  electrolytic  separation  of  manganese  from  other  metals, 
e.g.,  copper,  cobalt,  nickel,  zinc,  etc. 

Other  papers  which  were  published  at  that  time  by  Luckow, 
F.  W.  Clarke,  and  J.  B.  Haunay  described  the  electrolytic  deter- 
mination of  mercury,  which  was  found  to  separate  readily  from 
solutions  of  the  chloride  and  sulphate. 

A  method  for  the  electrolytic  determination  of  cadmium  was 
found  by  F.  W.  Clarke  (1878),  who  succeeded  in  precipitating  this 
metal  from  solutions  of  its  acetate,  and  Yver  (1880)  employed  a 
similar  solution  for  separating  cadmium  from  zinc. 

Cadmium  is  not  deposited  in  the  presence  of  nitric  acid  and  the 
attempt  was  made  by  Yver  to  separate  this  metal  from  copper, 
although  the  results  were  not  entirely  satisfactory. 

The  determination  of  zinc  from  solutions  of  the  double  cyanides 
was  carried  out  by  Beilstein  and  Jawein  (1879),  and  Fresenius 
and  Bergmann  (1880)  successfully  precipitated  metallic  nickel  and 
cobalt  from  solutions  containing  an  excess  of  free  ammonia  and 
ammonium  sulphate. 

Edgar  F.  Smith  showed  (1880)  that  if  uranium-acetate  solutions 
were  electrolyzed  the  uranium  was  completely  precipitated  as 
uranyl  hydroxide;  and,  further,  that  molybdenum  could  be 
deposited  as  hydrated  sesquioxide  from  warm  solutions  of  ammo- 
nium molybdate  in  the  presence  of  free  ammonia.*  We  are  in- 
debted to  the  same  author  and  his  students  for  a  large  number  of 
valuable  contributions  to  the  literature  of  electrochemical  analysis. 

Luckow  (1880)  rendered  a  special  service  in  the  publication  of 
his  observations  on  the  reactions  which  take  place  during  elec- 
trolysis. He  pointed  out  the  reduction  from  higher  to  lower  states 
of  oxidation  in  the  case  of  chromic  acid,  iron  and  uranium  salts, 
and  demonstrated,  on  the  other  hand,  that  sulphites  and  thio- 
sulphates  are  oxidized  to  sulphates.  He  summed  up  the  results 
of  his  observations  in  a  law,  that  in  general  the  electric  current 

*  M.  Heidenreich  could  not  obtain  good  results  by  this  method.  Ber.,  29, 
1587  (1896). 


104  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

exerts  a  reducing  action  on  acid,  and  an  oxidizing  action  on  alka- 
line, solutions.  Recent  investigations  have  shown,  however,  that 
other  factors  are  of  importance  in  these  reactions. 

In  the  year  1881,  Alexander  Classen  and  his  students  began  a 
series  of  investigations  on  quantitative  analysis  by  electrolysis 
which  ultimately  included  nearly  all  of  the  metals.  It  was  he  who 
first  pointed  out  the  value  of  oxalic  acid  and  the  double  oxalates. 
A  large  number  of  electrolytic  methods  originated  by  him  will  be 
described  in  this  book. 

At  about  the  same  time  Reinhardt  and  Ihle  proposed  the  double 
oxalates  for  the  electrolytic  determination  of  zinc. 

An  attempt  was  made  (1880)  by  Gibbs,  who  used  a  mercury 
cathode,  to  determine  metals  by  observing  the  increase  in  weight 
of  the  mercury  due  to  the  formation  of  an  amalgam,  and  a  similar 
method  was  employed  by  Luckow  (1886)  and  later  by  Paweck  for 
the  determination  of  zinc. 

The  mercury  cathode  has  been  recently  used  extensively  for 
determinations  and  separations  by  Kolloch  and  Smith  and  by 
Hildebrand. 

Since  the  year  1886,  a  great  number  of  publications  on  electro- 
chemical analysis  have  appeared,  and  it  is  unnecessary  to  enu- 
merate them  all.  Especially  worthy  of  mention  at  this  point, 
however,  are  the  experiments  conducted  by  Vortmann  (1894)  on 
the  electrolytic  determination  of  the  halogens  with  silver  anodes 
and  by  Specketer  (1899)  on  the  separation  of  the  halogens  in  a 
similar  way. 

The  investigations  of  Kiliani  (1883),  on  the  significance  of  the 
electromotive  force  in  electrolytic  determinations,  served  to  draw 
attention  to  this  important  factor,  and  the  later  work  of  Le  Blanc 
(1889)  on  the  electromotive  forces  necessary  for  the  decomposition 
of  solutions  of  the  salts  of  various  metals  added  greatly  to  the 
available  theoretical  data.  In  1891  Freudenberg  successfully 
separated  a  number  of  metals  from  solutions  containing  several 
metals  by  carefully  regulating  the  electromotive  force  of  the  cur- 
rent which  he  employed. 

Hand  in  hand  with  the  working  out  of  electrolytic  methods, 
improvements  were  made  in  the  apparatus.  The  laboratory  at 
Aachen  played  an  important  part  in  the  introduction  of  electrolytic 
appliances  and  devices.  As  source  of  current,  dynamos  and 
storage  cells  were  used  here  at  a  comparatively  early  date. 


HISTORICAL  105 

The  application  of  electro-analysis  has  recently  experienced  a 
revolution  through  the  introduction  of  rapid  electrolytic  methods. 
The  first  step  in  this  direction  was  evidently  based  upon  the 
suggestion  of  v.  Klobukow,  in  1886,  to  stir  the  electrolyte  in  order 
to  hasten  the  deposition  of  the  metal. 

In  1897,  A.  Classen  recommended,  in  the  fourth  German  edition 
of  this  book,  the  stirring  of  the  electrolyte  in  order  to  hasten  the 
deposition  of  copper.  In  1903,  Dr.  Amberg  attempted  to  deter- 
mine the  atomic  weight  of  palladium  by  electrolytic  measure- 
ments, but  as  he  was  unsuccessful  with  stationary  electrolytes, 
A.  Classen  suggested  rotating  one  of  the  electrodes  and  this  led 
to  the  desired  end. 

Since  1903,  a  great  deal  of  similar  work  has  been  done  inde- 
pendently by  American,  English  and  German  investigators: 
Acree,  Ashbrook,  Cutcheon,  Dennis,  Exner,  A.  Fisher,  Flanigen, 
Frary,  Gooch  and  Medway,  Hildebrand,  Ingham,  Langness, 
Lukens,  Pawek,  Perkin,  Price  and  Judge,  Sand,  Shepherd,  E.  F. 
Smith,  R.  O.  Smith  and  others.  Their  results  will  be  mentioned 
in  connection  with  the  individual  methods  described. 

In  1907  A.  Classen  described  an  outfit  for  carrying  out  rapid 
electrolytic  determinations,*  which  permitted  the  simultaneous 
carrying  out  of  a  number  of  electro-analyses  of  various  types. 

Such  is,  in  brief,  the  history  of  the  development  of  methods 
used  in  quantitative  analysis  by  electrolysis.f  A  short  account 
will  now  be  given  of  the  progress  of  chemical  theory  concerning 
electrolysis.! 

Grotthus,  in  1805,  explained  electrolysis  on  the  basis  of  a 
successive  decomposition  and  recombination  of  the  molecules 
of  the  electrolyte.  Thus,  when  water  was  subjected  to  elec- 
trolysis, a  molecule  of  water  was  decomposed  at  the  cathode  and 
hydrogen  was  evolved  as  a  gas.  The  oxygen  then  robbed  a  neigh- 
boring water  molecule  of  its  hydrogen  and  this  process  continued 
over  and  over  again  until  finally  at  the  anode  the  last  molecule  of 
water  decomposed  was  unable  to  find  hydrogen  from  any  other 
molecule  of  water  and  free  oxygen  was  evolved. 

*  Z.  Elektrochem.,  13,  181. 

t  A  detailed  account  of  all  typical  reactions  which  have  been  developed 
with  the  aid  of  electrolysis  can  be  found  in  F.  Foerster's  very  valuable  book, 
Elektrochemie  wasseriger  Losungen,  2nd  Edition,  Leipsic  (1915). 

t  In  preparing  this  outline,  much  of  the  information  has  been  obtained 
from  F.  J.  Moore's  History  of  Chemistry,  New  York,  1918. 


106  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Sir  Humphrey  Davy  (1778-1829)  was  of  the  opinion  that 
chemical  affinity  could  be  explained  on  the  basis  of  electrical 
attraction.  Previously  chemists  had  compared  chemical  affinity 
to  gravitation.  Davy  believed  that  when  two  atoms  capable 
of  combining  with  one  another  are  brought  near  one  another 
they  assume  opposite  electrical  charges  which  are  neutralized 
when  the  chemical  compound  is  formed.  Electrolysis,  according 
to  this  idea,  served  to  give  back  to  parts  of  the  original  molecule 
the  charges  possessed  before  combination  took  place. 

Michael  Faraday  (1791-1867)  also  believed  that  chemical 
affinity  is  of  electrical  nature.  He  understood  clearly  the  rela- 
tions that  exist  between  the  quantity  of  electricity  required  to 
deposit  a  given  weight  of  metal  and  considered  the  weights  de- 
posited as  the  best  criterion  for  the  determination  of  atomic 
weights.  He  believed  that  only  salts  consisting  of  a  positive 
and  a  negative  atom  are  electrolytes.  Faraday  introduced  the 
terms  anode,  cathode,  anion  and  cation.  He  believed,  *however, 
that  molecules  were  decomposed  into  anions  and  cations  by  the 
action  of  the  electric  current.  Faraday  believed  that  the  voltage 
required  to  effect  electrolysis  was  a  measure  of  chemical  affinity. 

Berzelius  (1779-1848)  placed  special  weight  upon  oxygen  and 
electricity  in  the  development  of  his  chemical  theory.  He  as- 
signed to  every  atom  two  poles  like  those  of  a  magnet.  For  any 
given  atom  the  positive  charge  at  one  pole  was  usually  unequal 
to  the  negative  charge  at  the  other  pole,  so  that,  with  the  exception 
of  hydrogen,  which  was  regarded  as  practically  neutral,  each 
element  was  more  or  less  positive  or  negative  in  nature.  Berzelius 
arranged  the  elements  in  a  series,  much  as  we  do  to-day,  placing 
potassium  at  the  positive  end,  oxygen  at  the  negative  end,  and 
hydrogen  in  the  middle.  Oxygen,  according  to  Berzelius,  was 
absolutely  negative.  Every  compound  substance  was  believed 
capable  of  being  resolved  into  two  parts,  one  electro-positive  in 
nature  and  the  other  electro-negative.  Salts  were  composed  of 
an  oxide  of  a  metal  and  an  oxide  of  a  non-metal;  in  the  former 
the  positive  charge  of  the  metal  predominated  and  the  oxide 
was  positive  in  nature,  but  in  the  latter  the  negative  character 
of  the  oxygen  was  not  overcome.  According  to  this  dualistic 
theory,  the  electric  current  resolves  a  salt  info  positive  and  negative 
oxides  which  appear  as  primary  products  at  the  cathode  and 
anode  respectively.  According  to  Berzelius,  an  acid  is  not  de- 


HISTORICAL  107 

composed  by  the  current  but  merely  serves  to  increase  the  con- 
ductance of  water  which  is  itself  decomposed  into  hydrogen  and 
oxygen.  Potassium  sulphate,  according  to  the  dualistic  theory, 
would  be  written,  K^O-SOs.  During  the  electrolysis  of  a  solu- 
tion of  this  salt,  K20  would  be  the  primary  product  at  the  cathode 
and  SOs  the  primary  product  at  the  anode.  Both  of  these  are 
hydrated  by  the  water  and  potassium  hydrate,  K^O  •  H^O  remains 
in  solution  at  the  cathode  and  SOs-H^O  remains  dissolved  at 
the  anode  as  final  products  of  the  electrolysis.  Zinc  sulphate, 
ZnO-SOs,  on  being  subjected  to  electrolysis  in  aqueous  solution 
is  decomposed  into  zinc  at  the  cathode  and  oxygen  at  the  anode; 
this  Berzelius  explained  by  assuming  that  the  ZnO  was  decomposed 
instead  of  the  entire  salt. 

In  1851  Williamson  advanced  the  idea  that  atoms  and  mole- 
cules in  compounds  exist  in  a  state  of  dynamic  equilibrium.  A 
molecule  instead  of  being  a  rigid  structure  was  always  exchanging 
material  with  neighboring  molecules.  Clausius  in  1857  applied 
this  idea  to  the  theory  of  electrolysis.  According  to  his  view 
the  electric  current  could  either  favor  or  hinder  such  an  exchange 
of  material  between  adjacent  molecules.  If  the  decomposed 
molecules  can  follow  the  electric  force  in  their  movement  then 
the  decomposition  will  be  favored. 

Hittorf  in  1853  conducted  a  remarkable  series  of  experiments 
on  the  rates  at  which  the  ions  moved  toward  the  poles  during 
electrolysis.  He  found  that  the  velocity  at  which  the  cation 
moves  toward  the  cathode  is  usually  different  than  the  rate  at 
which  the  anion  moves  toward  the  anode.  The  ratio  of  the 
velocity  of  one  ion  to  the  sum  of  the  velocities  of  both  ions  he 
termed  the  transference  number.  Hittorf  concluded  that  elec- 
trolytes are  salts  and  that  the  electrolyte,  not  the  solvent,  carries 
the  current. 

Kohlrausch  in  1876  confirmed  the  work  of  Hittorf  and  showed 
that  the  conductance  could  be  calculated  additively  from  the 
mobilities  of  the  ions.  Arrhenius  in  1887  drew  the  conclusion 
that  in  any  conducting  solution  only  a  certain  part  of  the  dis- 
solved substance  is  responsible  for  the  conductivity.  He  explained 
various  abnormalities  which  had  been  noticed  in  the  physical 
properties  of  solutions  by  assuming  that  ions  were  formed  as 
soon  as  the  electrolyte  was  dissolved  in  water.  Previous  investi- 
gators had  thought  that  the  current  split  the  molecule  into  ions. 


108          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

The  views  of  Arrhenius  concerning  electrolytic  dissociation,  or 
ionization,  are  now  accepted  by  most  chemists. 

Since  1887  important  progress  has  been  made  in  the  physical 
conception  of  the  ultimate  composition  of  matter.  In  1879 
Sir  William  Crookes  had  noticed  that  when  an  electric  current 
of  high  potential  was  passed  through  evacuated  tubes  con- 
taining gases  at  very  low  pressures,  rays  are  emitted  from 
the  cathode  of  remarkable  nature.  These  cathode  rays  proceed 
in  straight  lines  but  their  path  can  be  deflected  by  means  of  a 
magnet.  Rontgen,  in  1895,  found  that  when  cathode  rays 
impinge  against  a  solid  a  new  ray  is  generated  which  can  pene- 
trate material  that  is  opaque  to  ordinary  light  rays.  These 
rays  were  called  x-rays.  In  1896,  Becquerel  discovered  that 
similar  rays  could  be  obtained  from  uranium  salts  and  in  working 
over  pitchblende,  the  well-known  uranium  ore,  Madame  Curie, 
aided  by  her  husband,  P.  Curie,  and  G.  Bemont,  discovered  the 
element  radium  in  1898.  Radium  was  given  its  name  because 
of  the  intensity  of  the  radio-active  emanation  which  it  yields. 
Radium  spontaneously  emits  rays  of  three  different  types:  (1) 
a-rays  which  have  proved  to  be  positively  charged  helium  atoms, 
(2)  0-rays,  which  are  identical  with  the  cathode  rays  noticed  by 
Crookes,  and  (3)  7-rays  which  resemble  the  so-called  x-rays 
and  have  a  very  high  penetrative  force. 

In  addition  an  inert  gaseous  element,  niton,  is  evolved.  Thus 
radium,  a  chemical  element  of  high  atomic  weight  (226)  is  con- 
tinually losing  matter  and  energy  and  as  a  result  of  this  decom- 
position more  stable  elements  of  lower  atomic  weight  are  formed. 

Since  1900  Rutherford  and  Soddy  have  studied  such  phenomena 
and  advanced  a  theory  of  atomic  disintegration.  Likewise  J. 
J.  Thompson  has  worked  out  the  so-called  electron  theory  to  account 
for  the  ultimate  composition  of  all  matter.  This  theory  is 
applicable  to  all  branches  of  chemistry  and  serves  to  explain 
in  simple  terms  the  changes  that  take  place  during  electrolysis. 

The  cathode  rays  detected  by  Crookes  apparently  consist  of 
minute  electro-negatively  charged  particles.  They  have  the 
smallest  mass  of  any  particles  yet  known  and  have  been  called 
corpuscles  or  electrons.  The  atom  of  an  element,  according  to 
Thomson,  instead  of  being  a  simple,  indivisible  mass  as  we  were 
taught,  is  really  quite  complex  in  nature  and  consists  of  an  as- 
semblage of  negative  electrons  held  together  by  a  positively  charged 


HISTORICAL  109 

nucleus.  The  positive  charge  balances  the  total  charge  on  all 
the  electrons  so  that  the  atom  itself  is  neutral.  The  approximate 
number  of  electrons  present  is  proportional  to  the  atomic  weight 
of  the  element,  the  hydrogen  atom  containing  either  one  or  a 
very  small  number  of  electrons.  The  electrons  are  regarded  as 
moving  with  high  velocities  in  orbits  within  the  atoms  and  they 
occupy  a  very  small  part  of  the  atom  as  a  whole. 

According  to  Rutherford,  the  atom  has  a  small  central  core 
of  positive  electricity  surrounded  by  electrons,  i.e.,  negative 
charges  of  electricity.  The  atom  also  contains  an  outer  system 
of  electrons  which  are  held  together  much  less  firmly  than  those 
of  the  inner  system.  The  ability  to  lose  one  or  more  electrons 
from  the  outer  system  gives  to  the  elements  its  chemical  valence 
and  accounts  for  its  general  chemical  and  physical  behavior. 
If  the  element  loses  one  or  more  electrons  from  the  inner  system 
it  becomes  changed  into  another  element  of  lower  order. 

According  to  the  electron  theory,  electric  conduction  in  a  solid 
conductor  is  due  to  a  movement  of  the  electrons  of  the  outer 

• 

system.  This  means  that  the  current  flows  in  exactly  the  opposite 
direction  than  that  which  has  been  assumed  arbitrarily.  Instead 
of  the  current  moving  in  what  we  are  accustomed  to  call  the 
positive  to  negative  direction,  in  reality  only  negative  electricity 
moves  and  that  in  the  direction  which  we  arbitrarily  call  from 
a  lower  to  a  higher  potential.  In  other  words,  chemists  formerly 
fastened  their  attention  upon  the  positive  electricity.  We  now 
know  a  great  deal  more  about  negative  electricity  than  we  do 
about  positive  electricity,  and  in  fact,  the  actual  existence  of 
positive  electricity  has  been  questioned. 

The  electron  theory  easily  accounts  for  chemical  combination. 
An  element  can  lose  one  or  more  of  its  electrons  from  the  outer 
system,  provided  it  can  find  some  other  element  to  accept  it. 
Thus  the  element  sodium  can  give  up  one  of  its  electrons  to  another 
element,  such  as  chlorine.  The  sodium  atom  then  possesses  a 
unit  positive  charge  simply  because  it  has  lost  negative  electricity. 
The  chlorine,  originally  neutral,  has  become  negatively  charged 
in  virtue  of  its  having  accepted  an  electron  from  the  sodium 
atom.  The  chemical  attraction  that  holds  the  sodium  and 
chlorine  together  in  the  molecule  of  sodium  chloride  is  merely 
the  attraction  of  the  positively  charged  sodium  for  the  negatively 
charged  chlorine. 


110  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

The  tenacity  with  which  an  element  holds  on  to  the  electrons 
in  the  outer  system  varies  with  different  elements.  The  unit 
charge  or  quantity  of  electricity  lost  by  all  univalent  elements 
is  the  same  but  all  kinds  of  energy  may  be  resolved  into  two 
factors,  the  intensity  factor  and  the  capacity  factor.  Variations 
in  the  intensity  factor  of  chemical  energy  account  for  the  different 
degrees  of  chemical  affinity. 

The  valence  of  an  element  is  determined  by  the  number  of 
electrons  it  can  lose  or  can  accept  in  the  outer  system.  An 
element  which  ordinarily  has  a  positive  valence  of  two  has  the 
power  of  losing  two  electrons;  an  element  which  ordinarily 
has  a  negative  valence  of  two  has  the  power  of  holding  quite 
firmly  in  the  outer  system  two  negative  electrons  more  than 
that  corresponding  to  the  neutral  condition. 

When  acids,  bases  and  salts  are  dissolved  in  water  they  break 
down  to  a  certain  extent  into  positively  charged  cations  and 
negatively  charged  anions.  The  electron  theory  makes'  it  pos- 
sible to  understand  where  these  charges  originate.  The  ioniza- 
tion  of  the  neutral  molecule  also  takes  place  in  melted  salts  and 
can  be  detected  in  solids  at  temperatures  below  the  melting-point. 

Oxidation  and  reduction  processes  are  easily  explained  by  the 
electron  theory.  An  element  is  said  to  be  oxidized  whenever 
the  atom  is  made  to  lose  one  or  more  electrons,  an  element  is 
said  to  be  reduced  whenever  the  atom  accepts  one  or  more  elec- 
trons. Thus  when  the  atom  of  iron  is  made  to  lose  two  elec- 
trons, the  iron  is  oxidized  to  ferrous  salt  and  when  it  is  made 
to  lose  three  electrons  it  is  oxidized  to  ferric  salt.  The  permanga- 
nate anion,  Mn04~~,  is  composed  of  four  atoms  of  oxygen,  each 
bearing  a  double  electro-negative  charge  and  one  atom  of  man- 
ganese which  has  lost  seven  electrons.  In  contact  with  ferrous 
ions  the  atom  of  manganese  is  able  to  cake  away  one  electron 
from  each  of  five  atoms  of  iron  whereby  the  iron  is  oxidized  to 
the  ferric  condition  and  the  manganese  is  reduced  (because  it  has 
accepted  negative  electrons)  to  manganous  salt: 

Mn04~  +  5Fe++  +  8H+  =  Mn++  +  5Fe+++  +  4H20. 

In  the  cases  thus  far  considered  the  polyvalent  elements  lost 
or  gained  electrons  to  correspond  to  their  valence  numbers.  In 
many  compounds,  however,  the  atom  is  positive  toward  certain 
constitutents  in  the  molecule  and  negative  toward  others.  In 


HISTORICAL  111 

the  ammonia  molecule,  for  example,  the  nitrogen  has  accepted 
an  electron  from  each  of  three  hydrogen  atoms.  When  ammonia 
combines  with  hydrochloric  acid  to  form  ammonium  chloride, 
NEUCl,  the  nitrogen  has  combined  with  an  additional  atom  of 
hydrogen,  and  thereby  gained  an  electron,  and  also  combined 
with  a  negative  chlorine  atom,  thereby  losing  an  electron.  The 
nitrogen  in  ammonium  chloride,  therefore,  has  gained  electrons 
from  four  hydrogen  atoms  and  lost  an  electron  to  the  chlorine 
atom.  The  valence  of  the  nitrogen  in  ammonium  chloride  is 
five,  but  of  these  five  charges  four  are  negative  and  one  is  positive. 
Ammonium  chloride,  therefore,  belongs  to  the  same  state  of 
oxidation  as  ammonia.  This  is  sometimes  expressed  by  saying 
that  the  polarity  of  the  nitrogen  remains  —  3  in  ammonium 
chloride. 

The  polarity  of  any  ion  containing  more  than  one  atom  is 
the  algebraic  sum  of  the  valences  of  the  atoms  it  contains.  Thus 
as  oxygen  in  nearly  all  of  its  compounds  has  a  negative  valence  of 
two,  it  is  apparent  that  the  valence  of  manganese  in  MnO4~  is 
+  7,  of  nitrogen  in  NOs~  is  +  5  and  of  chlorine  in  C1O3~  is  +  5. 
The  application  of  this  rule  to  ions  containing  more  than  one 
atom  of  a  given  element  may  lead  to  confusion.  Thus  it  would 
seem  that  the  valence  of  the  carbon  in  the  oxalate  ion,  6264 =  is 
+  3.  The  graphic  symbol  for  sodium  oxalate,  however,  is 

O=C— O— Na 
O=C— 0— Na, 

and  it  is  clear  that  each  carbon  has  a  valence  of  four.  On  the 
other  hand,  it  is  reasonable  to  assume  that  the  valence  of  the 
carbon  atom  is  positive  toward  the  atoms  of  oxygen  but  obviously 
one  carbon  is  positive  to  the  other  carbon  atom  if  it  is  assumed 
that  one  end  of  each  valence  bond  is  positive  and  the  other  end 
negative.  Hence  one  atom  of  carbon  in  oxalic  acid  has  a  pos- 
itive valence  of  four,  but  the  other  atom  of  carbon  has  a  positive 
valence  of  three  and  a  negative  valence  of  one.  The  polarity 
of  the  two  atoms  of  carbon  =  is +4  +  3—  1  =  +6. 

We  may  say  that  in  oxalic  acid  the  average  polarity  of  the 
carbon  atom  is  +3.  When  oxalic  acid  is  heated,  it  decomposes 
into  water,  carbon  monoxide,  and  carbon  dioxide.  Evidently 


112  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

the  atom  of  carbon  in  the  carbon  monoxide  is  the  atom  which 
was  negative  to  the  other  carbon  atom  in  the  oxalate  molecule. 

The  electron  theory  offers  a  very  simple  explanation  of  the 
decompositions  that  take  place  during  electrolysis.  At  the  cathode 
negative  electrons  enter  the  solution  and  serve  to  accomplish 
a  chemical  reduction  there.  At  the  anode,  electrons  leave  the 
solution  and  pass  to  the  electrode  whereby  an  oxidation  is  accom- 
plished in  the  solution.  We  may  express  the  unit  charge  of  an 
electron  by  a  small  Greek  letter  e  enclosed  in  a  circle,  ©.  When 
the  electric  current  is  passed  through  a  solution  of  sodium  sul- 
phate, the  sodium  ions  migrate  toward  the  cathode  and  the 
sulphate  ions  toward  the  anode.  In  this  way  the  electric  current 
passes  from  pole  to  pole.  At  each  electrode,  however,  it  is  easier 
to  decompose  water  than  to  discharge  either  sodium  or  sulphate 
ions.  The  reaction  that  takes  place  at  the  cathode,  therefore, 
may  be  expressed  as  follows  : 

2H20  +  2©=  20H-  +  H2. 
The  reaction  that  takes  place  at  the  anode  is 
2H20  +  4© 


Two  molecules  of  hydrogen  are  set  free  at  the  same  time  one 
molecule  of  oxygen  is  liberated. 

In  1895  Ostwald  published  a  paper  on  "  The  Overthrow  of 
Scientific  Materialism."  He  pointed  out  that  all  we  know  in 
the  universe  concerns  changes  in  energy.  According  to  Ostwald, 
energy  is  the  only  reality  and  matter  is  merely  hypothetical. 
Most  of  us,  however,  cannot  conceive  of  energy  except  asso- 
ciated with  matter  and  cannot  think  of  matter  except  associated 
with  energy.  Ostwald,  however,  did  a  distinct  service  to  chemical 
science  in  pointing  out  that  we  really  know  more  about  energy 
than  we  do  about  matter  and  in  emphasizing  the  fact  that  every 
chemical  change  is  associated  with  a  transference  of  energy. 
The  electron  theory  does  not  tell  us  much  about  positive  elec- 
tricity, but  it  explains  the  possibility  of  an  atom  losing  its  identity 
merely  as  a  result  of  losing  energy  from  its  inner  system  and 
it  explains  how  an  element,  such  as  manganese,  may  show  entirely 
different  properties  as  a  result  of  gaining  or  losing  electrons. 


PART  II. 

ELECTRO-ANALYTICAL  DETERMINATIONS. 

IT  is  customary  and  convenient  in  the  study  of  methods  of 
analytical  chemistry  to  divide  the  elements  into  groups.  Thus 
in  qualitative  analysis  the  metals  are  divided  into  groups  on 
the  basis  of  the  solubilities  of  their  chlorides,  sulphides,  hydrox- 
ides and  carbonates;  likewise  the  acids  have  been  classified  on 
the  basis  of  their  volatility  and  the  solubility  of  their  silver  and 
barium  salts.  Practically  the  same  classification  may  be  followed 
to  advantage  in  the  study  of  ordinary  gravimetric  analysis.  Ti- 
tration  methods,  on  the  other  hand,  are  usually  divided  into 
reactions  of  acidimetry  and  alkalimetry,  reactions  of  oxidation 
and  reduction,  and  reactions  of  precipitation. 

The  reactions  of  electrolysis  always  involve  a  chemical  reduc- 
tion at  the  cathode  and  a  chemical  oxidation  at  the  anode.  Most 
of  the  methods  discussed  in  this  book  depend  upon  cathodic 
reduction.  Most  of  them  involve  the  quantitative  determina- 
tion of  a  metal. 

Any  satisfactory  classification  of  electro-analytical  methods, 
must  take  into  consideration  the  relative  ease  with  which  the 
metals  are  reduced  at  the  cathode  or,  in  other  words,  their  position 
in  the  electromotive  series  (page  26).  To  deposit  a  metal  upon 
the  cathode  it  is  necessary  to  overcome  the  oxidation  potential 
of  the  metal.  Other  things  being  equal,  the  lower  a  metal  stands 
in  the  potential  series,  the  easier  it  is  to  deposit  the  metal  upon 
the  cathode. 

It  has  been  pointed  out  repeatedly,  however,  that  the  relative 
position  of  the  elements  in  the  electromotive  series  is  not  always 
the  same.  Many  elements  can  exist  in  aqueous  solutions  in 
more  than  one  state  of  oxidation.  The  oxidation  potential  of 
iron  against  a  solution  of  a  ferrous  salt  is  greater  than  that  of 
iron  against  a  solution  of  a  ferric  salt  of  the  same  concentration. 
Moreover,  if  the  metal  exists  in  solution  in  the  form  of  a  com- 
plex ion  this  has  a  very  marked  effect  upon  the  oxidation  potential 
of  the  element  against  the  solution.  Thus  the  oxidation  potential 

113 


114  QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 

of  iron  against  a  solution  of  potassium  ferrocyanide  is  much  greater 
than  that  of  iron  against  a  solution  of  ferrous  sulphate  containing 
the  same  quantity  of  iron.  For  these  reasons  a  rigid  classification 
of  electrolytic  methods  of  analysis  in  accordance  with  the  elec- 
tromotive series  does  not  work  out  perfectly.  It  also  has  the 
disadvantage  of  placing  certain  of  the  rarer  elements  among 
those  to  be  considered  first.  The  order  in  which  the  methods 
will  be  discussed  in  this  book  will  be  based  partly  upon  theo- 
retical considerations  and  partly  upon  practical  grounds.  The 
elements  will  be  classified  into  the  following  groups :  * 

GROUP  I.  Metals  which  are  electro-negative  to  hydrogen  and 
can  be  deposited  quantitatively  on  the  cathode  from  acid  solu- 
tions. The  elements  in  tKis  grouo  are  copper,  silver,  mercury, 
gold,  palladium,  rhodium,  platinum,  (iridium),  bismuth,  anti- 
mony, tin,  (arsenic), f  tellurium,  (selenium).  Copper  will  be 
considered  first  because  this  element  has  been  determined  electro- 
lytically  more  than  any  other  element  and  methods  of  great 
accuracy  have  been  perfected  for  its  determination  to  which 
it  will  be  convenient  to  refer  in  considering  the  determination 
of  other  elements. 

GROUP  II.  The  metals  indium,  cadmium  and  zinc.  The 
exact  position  of  indium  in  the  series  is  not  known.  Cadmium 
and  zinc  are  above  hydrogen  in  the  electromotive  series,  but 
these  elements  can  be  deposited  upon  the  cathode  from  dilute 
acid  solution  owing  to  the  overvoltage  which  hydrogen  shows 
toward  them. 

GROUP  III.  The  metals  iron,  nickel,  and  cobalt.  It  is  prac- 
tically impossible  to  deposit  these  metals  quantitatively  unless 
the  concentration  of  the  hydrogen  ions  in  solution  is  kept  very 
low,  as  in  the  case  of  a  little  oxalic  acid  in  the  presence  of  a  large 
excess  of  alkali  oxalate.  As  a  rule  these  metals  are  precipitated 
from  an  alkaline  solution. 

GROUP  IV.  Metals  which  are  deposited  as  oxide  upon  one 
of  the  electrodes.  These  elements  are  lead,  thallium,  manganese, 
chromium,  molybdenum,  uranium  (tungsten,  vanadium,  nio- 
bium, and  tantalum). 

*  Cf.  A.  Fischer,  Electroanalytische  Schnellmethoden,  Stuttgart,  1908. 

t  An  element  in  parentheses  signifies  that  the  element  belongs  in  this 
group,  but  no  satisfactory  electrolytic  method  for  its  determination  will 
be  discussed. 


ELECTRO-ANALYTICAL  DETERMINATIONS  115 

GROUP  V.  Strongly  electro-positive  metals  which  cannot  be 
deposited  even  from  alkaline  solutions  except  in  the  form  of 
amalgams.  This  group  includes  (aluminium,  glucinum,  and 
rare  earths),  calcium,  strontium,  barium,  potassium,  sodium 
and  (ammonium). 

GROUP  VI.  Metalloids  and  anions  which  undergo  anodic 
oxidation.  Fluorine,  chlorine,  bromine,  iodine,  sulphur,  car- 
bonate, ferrocyanide,  phosphate  and  nitrate  anions,  etc. 

First  the  electrolytic  methods  will  be  discussed  on  the  assump- 
tion that  no  other  metal  likely  to  interfere  is  present  in  the 
solution.  Then,  after  all  the  groups  have  been  considered,  some 
separations  will  be  described  with  special  reference  to  electro- 
lytic methods  that  have  been  found  useful  in  commercial  prac- 
tice. 

Electrolytic  work  is  capable  of  yielding  very  accurate  results, 
but  often  a  slight  change  in  the  conditions,  such  as  size  and  shape 
of  the  electrodes,  volume  of  the  solution,  the  acidity  or  the  tem- 
perature will  cause  trouble  so  that  it  is  advisable  to  follow  directions 
very  closely.  Many  of  the  methods  have  been  worked  out  before 
the  theory  of  electrolysis  was  well  understood  and  emphasis 
was  placed  upon  relatively  unimportant  conditions.  For  this 
reason,  in  describing  the  methods  of  various  investigators  there 
will  be  some  duplication  of  data  and  in  some  cases  apparent 
contradiction. 

Important  general  data  concerning  each  element  will  be  given. 
This  will  include  the  atomic  weight  (At.  Wt.),  the  electro-chemical 
equivalent  (Elec.  Equiv.),  or  weight  deposited  by  one  ampere  in 
one  second,  the  electrolytic  or  oxidation  potential  (Elec.  Po- 
tential), of  the  element  referred  to  the  normal  hydrogen  electrode 
as  of  zero  potential,  and  the  overvoltage  which  hydrogen  shows 
against  a  cathode  of  the  metal  in  question.  In  giving  the  values 
of  the  oxidation  potential,  a  positive  sign  will  indicate  that  the 
element  is  above  hydrogen  in  the  potential  series. 


116  QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 

GROUP  I. 

METALS  ELECTRO-NEGATIVE  TO  HYDROGEN. 
Copper. 

At.  Wt.  =  63.6.  Elec.  Equiv.  =  0.328  mg.  Elec.  Potential  = 
-0.34  volt  for  Cu  +  +  ions.  Overvoltage  of  H2  =  0.03-0.23 
volt. 

AT  least  six  distinct  methods  have  been  proposed  for  the  electro- 
lytic determination  of  copper.  (1)  The  analysis  is  carried  out  in  a 
sulphuric-acid  solution;  (2)  in  a  nitric-acid  solution;  (3)  in  an 
ammoniacal  solution;  (4)  in  an  alkalicyanide  solution;  (5)  in  an 
acid-oxalate  solution;  (6)  in  a  phosphate  solution. 

Only  the  first  three  of  these  methods  will  be  discussed  in  de- 
tail. The  deposition  of  copper  from  an  alkalicyanide  solution  is 
useful  hi  the  separation  of  this  metal  from  iron,  molybdenum, 
platinum,  palladium  and  selenium.  The  use  of  a  complex  copper 
oxalate  as  electrolyte  offers  no  special  advantages  for  the  quantita- 
tive determination.  Such  an  electrolyte  is  excellent,  however, 
when  it  is  desired  to  obtain  quickly  a  dense,  glistening  deposit  of 
copper,  for  the  purpose  of  subsequently  determining  zinc.  The 
experiments  of  M.  Heidenreich  carried  out  in  the  author's  labora- 
tory have  shown  that  the  deposition  of  copper  from  a  phosphate 
solution  is  not  to  be  recommended. 

1 .  Deposition  of  Copper  from  Sulphuric-acid  Solution. 
According  to  the  older  methods  for  carrying  out  the  electrolysis 
in  a  sulphuric-acid  solution,  it  was  necessary  to  add  certain  sub- 
stances to  the  electrolyte.  When  a  solution  of  copper  sulphate 
in  dilute  sulphuric  acid  is  subjected  to  electrolysis  with  a  cur- 
rent having  a  certain  strength  at  the  start,  the  current  gradually 
diminishes  in  strength  as  the  copper  is  deposited  and  thus  it 
requires  a  very  long  time  for  the  removal  of  the  last  traces  of  the 
metal.  It  used  to  be  customary,  therefore,  to  turn  on  more 
current  toward  the  end  of  the  operation,  and  thus  the  work  was 
carried  out  with  a  current  of  practically  constant  strength.  In 
such  cases,  however,  the  cathode  potential  in  the  solution  im- 
poverished of  copper  ions  becomes  greater  than  the  discharge 
potential  of  hydrogen  ions  and  this  is  the  reason  why  the  last 


COPPER  117 

traces  of  copper  are  deposited  in  a  spongy  condition  (cf.  p.  22). 
It  was  found  possible  to  prevent  the  formation  of  a  spongy 
deposit  by  adding  one  of  a  number  of  different  substances,  such 
as  urea,  hydroxylamine,  nitric  acid,  or  ammonium  nitrate.  The 
reason  why  nitric  acid  or  a  nitrate  has  a  favorable  effect  is 
because  the  reduction  potential  of  the  nitrate  anion  is  lower  than 
the  discharge  potential  of  hydrogen;  the  anion  can  be  reduced 
to  ammonium  cations  or  to  free  ammonia  without  evolution  of 
hydrogen. 

The  reduction  of  the  nitrogen  from  its  positive  valence  of 
five  in  the  nitrate  anion  to  a  negative  polarity  of  three  (or  neg- 
ative valence  of  four  and  positive  valence  of  one)  can  be  expressed 
by  the  equation: 

N03-+  10  H+  +  8  ©  =  NH+  +  3H20. 

This  reaction  shows  that  the  acidity  of  the  solution  decreases 
rapidly  during  the  progress  of  the  electrolytic  reduction  of  the 
nitrate  anion.  When  the  hydrogen  ions  are  all  neutralized, 
free  ammonia  is  formed: 

N03~  +8©+  7H20  -»  NH+  +  10  OH^  NH3  +  9  OH~  +H20. 

The  neutralization  of  the  acid  may  cause  metals  to  precipitate 
as  hydroxides  or  to  deposit  upon  the  cathode  with  the  copper. 

Ammonium  ions  and  free  ammonia,  however,  are  not  the  only 
possible  products  from  the  electrolytic  reduction  of  nitrate  ions. 
Under  certain  conditions  considerable  hydroxylamine  is  formed 
and  often  an  appreciable  quantity  of  nitrous  acid.  Usually  very 
little  nitrous  acid  is  present  in  the  solution  at  any  one  time, 
because  it  is  reduced  farther  very  easily.  The  presence  of  any 
considerable  quantity  of  nitrous  acid  will  cause  a  copper  deposit 
to  dissolve  off  the  electrode  even  while  the  current  is  still  passing 
and  it  is  chiefly  due  to  the  presence  of  a  little  nitrous  acid  that 
special  precautions  are  often  necessary  in  removing  the  electrode 
from  the  solution  at  the  end  of  the  electrolysis  when  all  the 
copper  has  been  deposited. 

Careful  experiments  indicate  that  metallic  copper  is  not 
appreciably  soluble  in  cold,  dilute  nitric  acid  which  contains  no 
nitrous  acid.  If  nitrous  acid  is  present  copper  dissolves  very 
rapidly  and  fresh  nitrous  acid  is  constantly  formed  during  the 
progress  of  the  reaction. 


118  QUANTITATIVE  ANALYSIS  [BY  ELECTROLYSIS 

Cu  +  2NG>2-  +  4H+  ->  Cu++  +  2NO  +  2H20, 
HN03  +  2NO  +  H2O  ->  3HN02. 

It  is  possible  to  remove  nitrous  acid  by  adding  urea 

CO(NH2)2  +  2HN03  =  C02  +  2N2  +  3H20. 

Foerster's  method  of  carrying  out  the  electrolysis  makes  the 
addition  of  nitric  acid  superfluous  because  the  electrolysis  is  not 
carried  out  with  a  current  of  constant  amperage  but  rather  with 
one  of  constant  voltage.  When  the  potential  of  the  current  is 
kept  at  two  volts,  as  when  a  single  accumulator  cell  is  used  which 
has  this  potential  when  not  too  far  exhausted,  a  voltage  is  provided 
which  is  enough  higher  than  the  decomposition  potential  of  copper 
sulphate  to  effect  the  complete  deposition  of  the  copper,  while, 
on  the  other  hand,  the  overpotential  of  hydrogen  ions  toward  the 
copper  plated  on  the  electrode  is  so  large  that  there  is*  scarcely 
any  evolution  of  hydrogen  (cf.  p.  82).  There  is,  therefore, 
nothing  to  cause  the  copper  to  be  deposited  in  a  spongy  condition. 
There  is  no  reason  why  several  electrolytic  cells  should  not  be 
connected  together  in  parallel  and  be  simultaneously  fed  with 
the  current  of  two  volts. 

As  regards  the  accelerating  effect  obtained  by  heating  the 
solution,  this  is  partly  explained  by  the  fact  that  the  diffusion 
velocity  is  greater  in  the  hot  solution  than  in  the  cold,  and,  there- 
fore, the  copper  ions  are  carried  toward  the  cathode  with  greater 
rapidity.  The  higher  temperature  of  the  liquid  also  lessens  the 
overpotential  of  the  oxygen  at  the  anode.  For,  just  as  the  hydro- 
gen experiences  an  overvoltage  toward  the  metal  at  the  cathode,  so, 
in  the  same  way,  the  oxygen  experiences  a  similar  effect  at  the 
anode.  This  increase  of  anodic  potential  serves  to  lessen  the  cur- 
rent strength  and  thus  the  opposite  effect  is  obtained  by  heating 
the  solution  (see  also  p.  92). 

The  duration  of  the  electrolysis  cannot  be  shortened  indefinitely 
by  raising  the  temperature  above  80°.  It  is  a  well-known  fact 
that  metallic  copper  tends  to  form  a  small  quantity  of  dissolved 
cuprous  sulphate  in  accordance  with  the  equation, 

CuS04  +  Cu  =  Cu2SO4, 
or 

Cu++  +  Cu  =  2  Cu+. 


COPPER  119 

This  reduction  of  the  cupric  sulphate  may  take  place  at  the 
cathode  even  while  the  current  is  passing  through  the  solution 
and  the  higher  the  temperature  the  greater  the  tendency  for  the 
reduction  to  take  place.  The  reaction  is  thus  a  reversible  one, 


because  the  cuprous  sulphate  is  constantly  being  oxidized  back  to 
cupric  sulphate  by  the  oxygen  of  the  atmosphere  as  well  as  that 
of  the  anode,  so  that  the  above  equilibrium  expression  represents 
the  true  condition.  Thus,  while  a  part  of  the  current  is  being  used 
for  depositing  the  copper  at  the  cathode,  another  part  is  lost,  in 
consequence  of  the  reversible  process  just  mentioned,  for  the 
wasted  current  serves  only  to  effect  the  reduction  of  the  cupric  ions 
to  cuprous  ions.  There  would  be  no  loss  of  current  if  the  cuprous 
sulphate  were,  in  its  turn,  reduced  directly  to  metallic  copper. 
Since,  however,  there  is  a  tendency  to  form  cupric  ions  again,  there 
must  be  a  certain  amount  of  current  wasted  and  this  waste  of  elec- 
tricity becomes  greater  as  the  temperature  is  raised.  It  is,  there- 
fore, advisable  to  keep  the  hot  solution  between  70°  and  80°  rather 
than  to  heat  it  to  a  higher  temperature. 

Precise  directions  will  now  be  given  for  carrying  out  the  elec- 
trolytic determination  of  copper  in  sulphuric  acid  solution  by  two 
well-tested  methods. 

Procedure.  Method  A.*  Weigh  out  about  0.3  gm.f  of  the 
metal  into  a  beaker  of  about  150-cc.  capacity.  Cover  the  beaker 
with  a  watch-glass,  to  prevent  loss  by  spattering,  and  dissolve 
the  metal  in  10  cc.  of  6-normal  nitric  acid  with  gentle  heating. 
When  the  metal  is  all  dissolved,  wash  down  the  sides  of  the 
beaker  and  the  bottom  of  the  cover  glass  with  a  little  water,  add 
5  cc.  of  6-normal  sulphuric  acid  and  evaporate,  without  boiling, 
until  all  the  nitric  acid  is  expelled  and  heavy  fumes  of  sulphuric 
acid  are  evolved.  Cool,  dilute  to  100  cc.,  and  electrolyze,  pref- 
erably with  a  gauze  electrode  (page  67),  keeping  the  e.m.f.  of 
the  current  at  2  volts.  If  a  lead  accumulator  is  used,  connect 

*  F.  Foerster,  Z.  angew.  Chem.,  19,  1890  (1906);  Ber.,  39,  3029  (1906). 

t  For  practice,  about  1  gm.  of  blue  vitriol  (CuSO4  •  5H2O)  may  be  used. 
Dissolve  1  gm.  in  10  cc.  of  double-normal  sulphuric  acid  and  90  cc.  of  water. 
The  solution  is  then  ready  for  electrolysis.  Double-normal  sulphuric  acid 
contains  98  gins,  of  H2SO4  per  liter,  or  about  55  cc.  of  concentrated  H2SO4 
per  liter. 


120  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

the  cathode  with  the  negative  pole  (lead  plate)  and  the  anode 
with  the  positive  pole  (peroxide  plate). 

The  complete  deposition  of  the  copper  requires,  under  these 
conditions,  about  eight  hours,  and  it  is  convenient,  therefore,  to  let 
the  current  pass  through  the  solution  overnight.  The  end  of  the 
reaction  can  be  told  fairly  closely  by  the  marked  lessening  of  the 
oxygen  evolution  at  the  anode.  A  few  drops  of  solution  are  then 
removed,  with  the  aid  of  a  short  piece  of  glass  tubing,  transferred 
to  a'porcelain  tile  and  mixed  with  a  drop  of  potassium-ferrocyanide 
solution.  There  should  be  no  evidence  of  red  cupric  f errocyanide.  * 

The  time  required  for  the  analysis  can  be  shortened  considerably 
by  heating  the  electrolyte  to  70  or  80°  (keeping  a  small  flame  under 
the  beaker  until  the  analysis  is  finished) ;  in  this  way  from  0.15  to 
0.25  gm.  of  copper  is  deposited  in  from  60  to  80  minutes,  f 

If  the  deposition  took  place  at  the  room  temperature,  it  is 
simply  necessary,  at  the  end  of  the  operation,  to  disconnect  the 
current,  quickly  remove  the  cathode,  rinse  off  the  adhering  solu- 
tion with  a  stream  of  water  from  the  wash  bottle,  dip  the  electrode 
in  a  beaker  of  distilled  water  that  is  ready  at  hand,  then  in 
alcohol,  and  dry  it  in  the  air  bath  at  from  80  to  90°  before  weighing. 
The  total  weight  of  the  electrode  with  the  copper  upon  it  minus 
the  original  weight  of  the  electrode  gives  the  quantity  of  copper 
that  was  present  in  the  solution  electrolyzed. 

If,  however,  the  deposition  of  the  copper  took  place  from  a  hot 
solution,  the  current  must  not  be  disconnected  until  after  the 
washing  of  the  cathode  has  been  completed,  because  otherwise  the 
hot,  dilute  sulphuric  acid,  with  the  aid  of  atmospheric  oxygen,  will 
dissolve  considerable  copper  from  the  electrode. 

From  the  hot  solution,  withdraw  the  electrode  slowly  and  wash 
it  with  a  stream  of  water  from  the  wash  bottle  while  withdrawing 
it.  Do  not  disconnect  the  current  until  the  electrode  has  been 
withdrawn  from  the  solution. 

Finally,  wash  the  electrode  with  alcohol,  heat  in  the  drying 
oven  just  long  enough  to  evaporate  off  the  alcohol,  cool  in  a 
large  desiccator,  and  weigh. 

*  In  every  electrolysis,  the  solution  should  be  tested  at  the  end  to  see  if  all 
metal  has  been  deposited  within  the  limits  to  which  it  is  possible  to  detect  it 
qualitatively.  The  time  stated  in  the  above  directions  can  be  influenced  by  a 
number  of  factors  and  should  not  be  regarded  as  absolutely  accurate. 

t  Under  these  conditions  (temperature,  voltage  and  degree  of  acidity) 
copper  can  be  separated  from  large  quantities  of  nickel,  cadmium  and  zinc. 


COPPER  121 

In  case  there  is  doubt  whether  all  the  copper  has  been  deposited, 
clean  the  electrode  by  means  of  hot  dilute  nitric  acid  and  elec- 
trolyze  a  little  longer  to  see  if  any  further  deposit  is  formed.  If 
this  is  the  case,  its  weight  should  be  added  to  that  previously 
obtained. 

Procedure.  Method  B.  Dissolve  about  0.5  gm.  of  metal  in 
nitric  acid  and  evaporate  with  sulphuric  acid  as  in  Method  A. 
Cool,  dilute  to  100  cc.,  add  1  gm.  of  solid  ammonium  nitrate 
and  electrolyze  for  twenty  hours  with  a  current  of  0.1  ampere. 
At  the  end  of  this  time,  add  0.25  gm.  of  urea  and  a  little  water. 
If,  after  stirring  the  solution  and  allowing  the  current  to  pass 
for  half  an  hour  longer,  there  is  no  evidence  of  further  deposition 
of  copper,  carefully  remove  the  cathode,  as  directed  in  Method 
A  when  working  with  a  hot  solution,  wash  well  with  water,  rinse 
in  alcohol  and  dry  at  105°  for  a  few  minutes.  Cool  and  weigh. 
Clean  the  electrodes  with  nitric  acid  and  see  if  any  further  deposit 
of  copper  can  be  obtained;  or,  test  the  solution  for  copper  to  see 
if  a  blue  color  is  obtained  with  excess  of  ammonium  hydroxide.* 
In  case  a  blue  color  is  obtained,  make  the  solution  slightly  acid 
and  electrolyze  again. 

Rapid  Deposition  of  Copper  from  Sulphuric-Acid  Solutions.! 

No  fault  can  be  found  with  the  accuracy  of  the  electrolytic 
determination  of  copper  with  a  stationary  electrolyte  during  a 
period  of  from  12  to  24  hours.  It  is,  however,  often  desirable 
to  obtain  results  in  a  much  shorter  time.  Moreover,  if  a  large 
number  of  analyses  are  to  be  made  in  a  given  time  with  platinum 
electrodes,  the  expense  of  equipment  increases  as  the  time  re- 

*  Nickel  also  gives  a  blue  color  with  an  excess  of  ammonium  hydroxide. 
The  color  in  each  case  is  due  to  complex  ions;  e.g.,  [Cu(NH3)4]  +  +  . 

t  General  references  to  the  literature  concerning  the  rapid  electrolytic 
determination  and  separation  of  copper  from  various  solutions:  Gooch  and 
Medway,  Am.  J.  Sci.  [4],  16,  320  (1903);  Z.  angew  Chem.,  36,  414  (1903). 
Exner,  J.  Am.  Chem.  Soc.,  25,  896  (1903).  E.  F.  Smith,  ibid.,  p.  884.  A. 
Fischer  and  Boddaert,  Z.  Elecktrochem.,  10,  945  (1904).  D.  S.  Ashbrook, 
J.  Am.  Chem.  Soc.,  26,  1283  (1904).  E.  F.  Smith  and  Kollock,  ibid.,  27, 
1255  (1905).  Flanigen,  Thesis,  1906,  U.  Pa.,  Philadelphia.  Langness,  The- 
sis, 1906,  U.  Pa.,  Philadelphia.  Perkin,  Chem.  News,  93,  283  (1906);  Z. 
Elektrochem.,  13,  143  (1906).  H.  J.  S.  Sand,  J.  Chem.  Soc.,  London,  91, 
373  (1907);  Z.  Elektrochem.,  13,  326  (1907).  A.  Fischer,  Z.  angew.  Chem., 
20,  134  (1907);  Z.  Elektrochem.,  13,  469  (1907).  Frary,  Z.  Elektrochem., 
13,  308  (1907);  Z.  angew.  Chem.,  20,  1897  (1907). 


122  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

quired  for  electrolysis  is  lengthened.  For  these  reasons  a  great 
many  experiments  have  been  made  since  1900  in  the  study  of 
methods  requiring  less  time  than  those  described  above. 

Since  a  current  of  1  ampere  will  deposit  0.328  gm.  of  copper 
in  one  second,  or  1.181  gm.  in  an  hour,  it  is  clear  that  the  problem 
of  the  rapid  determination  of  copper  by  electrolysis  resolves 
itself  into  the  determination  of  conditions  under  which  current 
strengths  of  1  or  more  amperes  may  be  used  without  detriment 
to  the  character  of  the  deposit  and  of  conditions  under  which 
practically  all  of  the  current  will  be  utilized,  as  long  as  copper 
ions  remain  in  solution,  for  the  deposition  of  metal. 

As  already  pointed  out,  high  current  densities  are  likely  to 
give  spongy  deposits  because  the  natural  migration  of  the  ions 
does  not  take  place  fast  enough  to  keep  copper  ions  in  the  vicinity 
of  the  cathode;  a  time  soon  comes  when  it  is  easier  to  discharge 
hydrogen  ions  than  to  deposit  copper  from  the  solution  which 
has  become  impoverished  with  respect  to  copper  ions. 

In  general,  more  current  can  be  used  in  proportion  as  the 
solution  is  concentrated  and  the  electrode  surface  large.  In- 
creasing the  electrode  surface  by  diluting  the  solution  is  unde- 
sirable. 

A  platinum  gauze  electrode  (cf.  p.  67)  is  useful  because  a  large 
electrode  surface  is  obtained  in  proportion  to  the  weight  of  the 
electrode  and  because  the  meshes  of  the  gauze  permit  the  ready 
passage  of  the  electrolyte  when  it  is  stirred  by  convection  cur- 
rents or  otherwise.  Heating  the  solution  helps  by  accelerating 
the  rate  of  diffusion  but  when  a  strong  current  is  used  enough 
electrical  energy  is  transformed  into  heat  energy  to  raise  the 
temperature  of  the  solution,  sometimes  even  to  the  boiling-point. 
By  using  a  gauze  cathode  and  a  current  of  6  amperes,  J.  L.  Stod- 
dard  was  able  in  ten  minutes  to  get  a  good  deposit  and  com- 
plete deposition  of  the  metal  from  a  solution  containing  0.5  gm. 
of  copper  in  50  cc. 

Stirring  the  solution,  by  means  of  an  independent  stirrer,  by 
causing  either  the  anode  or  the  cathode  to  rotate,  or  by  means 
of  a  magnetic  effect  (cf.  p.  73)  has  also  proved  very  helpful. 
The  following  table  gives  a  summary  of  conditions  under  which 
good  results  have  been  obtained  by  various  analysts  with  stirred 
electrolytes. 

The  abbreviation  NDioo  used  in  this  table  and  elsewhere  sig- 


COPPER 


123 


nifies  the  amperage  per  100  sq.  cm.  of  electrode  surface  in  the 
electrolyte.  This  is  the  way  in  which  the  current  density  is 
usually  expressed. 


Experiments  performed  by 

A.  Fischer  at 
Aachen. 

Gooch  and 
Medway. 

Exner. 

H.  J.  S.  Sand. 

Kind  of  electrode  

Electrolyte  contained  . 
Volume  .  .  

Platinum 
dish  and 
rotating 
disk 

12  cc.  cone. 
H2S04 

125  cc. 

0.3  gm.  as 
sulphate 

55°-65° 
2.8-2.6  volts 

Rotating 
platinum 
crucible  as 
anode 

6  or  7  drops 
H2S04(1:4) 

50  cc. 

0.25  gm.  as 
sulphate 

Begun  in 
the  cold 

About  8 
volts 

Platinum 
dish  and 
rotating 
spiral 

1  cc.  H2SO4 
(1:10) 

125  cc. 

0.5  gm.  as 
sulphate 

Boiling 
14-9  volts 

5  amperes 
600 

3-5  minutes 

Sand's  elec- 
trode, p.  62 

0.75  cc.  to 
1  cc.  cone. 
H2S04 

85  cc. 

0.5  gm.  as 
sulphate 

Luke  warm 
or  boiling 

2.8-3  volts 

10  amperes 
300-600 

5-7  minutes 

Quantity  of  metal.  .  . 

Temperature  

Voltage  

Current  density,  NDioo 

Number  of  revolutions 
per   minute    of   the 
stirrer. 

Duration  . 

800 
33  minutes 

600-800 

10-15  min- 
utes 

Solenoid  Method.  G.  L.  Heath  recommends  the  following 
method  for  the  examination  of  samples  of  commercial  copper.* 
The  use  of  a  large  sample  is  advocated  in  order  that  the  results 
may  be  more  representative.  It  is  claimed  that  the  use  of 
a  mixture  of  nitric  and  sulphuric  acids  of  the  specified  concen- 
trations has  been  found  empirically  to  give  good  results  even 
when  0.5  per  cent  of  arsenic  is  present. 

Procedure.  Dissolve  5  gm.  of  metal  in  a  mixture  of  exactly 
7  cc.  of  nitric  acid,  sp.  gr.  1.42,  10  cc.  of  concentrated  sulphuric 
acid  and  25  cc.  of  water.  Use  a  lipless  beaker  of  300-cc.  capacity 
which  is  about  12  cm.  tall  and  of  0.5  cm.  diameter.  Cover  the 
beaker  with  a  watch  glass  and  heat  just  below  the  boiling-point 

*  J.  Ind.  Eng.  Chem.,  3,  77  (1911). 


124          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

until  the  copper  is  all  dissolved.  Rinse  off  the  moisture  that 
has  condensed  on  the  watch  glass  and  wash  down  the  sides  of 
the  beaker  with  a  stream  of  water  from  the  wash  bottle,  finally 
diluting  to  about  100  cc.  Place  the  beaker  in  the  solenoid  ap- 
paratus (cf.  p.  73  and  p.  74)  and  electrolyze  with  a  gauze  cathode, 
using  a  current  of  4.5  amperes  through  the  solution  and  through 
the  cofl.  Cover  the  beaker  with  a  pair  of  split  watch  glasses  to 
prevent  loss.  In  about  two  hours  and  a  half  the  solution  will 
become  colorless  and  the  copper  all  deposited.  It  is  important 
not  to  continue  the  analysis  much  longer  than  necessary  (cf.  p. 
117).  When  the  solution  has  become  colorless,  wash  the  bottom 
of  the  watch  glasses  and  the  sides  of  the  beaker  with  a  little  water 
and  continue  the  electrolysis  for  about  half  an  hour  longer.  Then 
withdraw  about  1  cc.  with  a  medicine  dropper  and  test  it  for 
copper  with  freshly-prepared  hydrogen  sulphide  water.  If  a 
negative  test  is  obtained,  quickly  transfer  the  cathode  to  a  beaker 
of  cold  water.  Then  turn  off  the  current,  wash  the  electrode 
well  with  water  and  finally  rinse  with  alcohol.  Dry  at  110  to 
120°  just  long  enough  to  evaporate  off  the  alcohol;  weigh 
when  cool. 

Deposition  of  Copper  from  Nitric-acid  Solution. 

The  deposition  of  copper  from  a  solution  containing  free  nitric 
acid  was  first  accomplished  successfully  by  Luckow  who  deter- 
mined in  this  way  small  amount  of  copper  in  the  Mansfeld  slates. 

Procedure.  For  the  analysis  of  a  copper  salt,  dissolve  about 
1  gm.  in  120  to  150  cc.  of  water.  Of  copper  wire,  dissolve  0.25 
gm.  in  4  or  5  cc.  of  nitric  acid,  sp.  gr.  1.2.  Dilute  the  solution 
to  120  cc.  and  boil  very  gently,  with  the  beaker  covered,  to  expel 
all  nitrous  fumes.  Then  rinse  off  the  cover  glass  and  wash  down 
the  sides  of  the  beaker.  To  the  solution  prepared  in  either  of 
these  ways,  add  2  or  3  cc.  of  nitric  acid,  sp.  gr.  1.2,  and  about 
0.1  gm.  of  urea  to  react  with  nitrous  acid,  in  case  any  is  present. 
Electrolyze  with  a  current  of  0.5  to  1  ampere  per  100  sq.  cm.  of 
exposed  electrode  surface,  using  either  a  platinum  dish,  platinum 
cylinder,  platinum  cone,  or  platinum  gauze  *  electrode.  Toward 

*A  fairly  satisfactory  gauze  electrode  can  be  made  from  copper  gauze 
such  as  used  in  the  determination  of  nitrogen  in  organic  substances  by  the 
Dumas  method.  It  is  necessary  to  make  sure  that  any  lacquer  or  oxide  is 
removed  before  using  such  an  electrode,  which  may  be  accomplished  by 


COPPER  125 

the  end  of  the  analysis  add  a  little  more  urea.  The  temperature 
of  the  solution  may  be  from  18  to  30°. 

As  regards  the  termination  of  the  electrolysis,  the  manipulation 
varies  a  little  with  the  nature  of  the  cathode.  When  the  platinum 
cone  or  gauze  electrode  is  used,  the  cathode  should  not  be  entirely 
covered  by  electrolyte,  although  it  must  reach  to  near  the  bottom 
of  the  beaker.  To  determine  whether  all  the  copper  has-  been 
removed  from  the  solution,  raise  the  level  of  the  liquid  a  few 
millimeters  by  mixing  a  little  water  with  the  solution  and  after 
some  time  has  elapsed  note  whether  there  is  any  deposit  formed 
on  the  freshly  exposed  surface  of  the  electrode.  If  this  is  the 
case,  the  electrolysis  must  be  continued.  If,  on  the  other  hand, 
there  is  no  further  deposit  of  copper  formed  after  ten  or  fifteen 
minutes,  remove  a  little  of  the  solution  and  test  with  potassium- 
ferrocyanide  solution  (see  p.  120).  If  a  platinum  dish  is  used  a? 
cathode,  it  should  be  only  about  two  thirds  full  at  the  start; 
there  is  then  enough  space  left  to  expose  a  fresh  platinum  surface 
by  diluting. 

On  account  of  the  solubility  of  copper  in  nitric  acid  it  is  not 
advisable,  when  most  accurate  results  are  desired,  to  remove 
the  cathode  in  the  simple  manner  described  on  page  120,  or,  in 
case  a  platinum  dish  is  used,  to  simply  pour  out  the  electrolyte 
and  rinse  it  with  water;  when  nitric  acid  is  present  the  wash- 
ing should  be  effected  before  the  circuit  is  broken.  To  ac- 
complish this,  allow  distilled  water  to  run  slowly  into  the  cell 
through  rubber  tubing  from  a  bottle  placed  above  it.  Cause 
the  water  to  flow  against  the  sides  of  the  beaker  or  dish,  and, 
while  the  water  is  being  added,  draw  off  the  original  contents 
of  the  cell  through  a  siphon  leading  from  the  bottom  of  the 
vessel.  By  means  of  a  pinchcock  on  the  rubber  tubing,  it  is 
easy  to  regulate  the  flow  of  water  into  the  vessel  and  by  join- 
ing some  rubber  tubing  to  the  siphon,  it  is  also  possible  to 

heating  and  then  plunging  the  electrode  into  a  large  test  tube  containing  a 
little  methyl  alcohol  at  the  bottom.  Care  should  be  taken  not  to  melt  the 
wire  during  the  heating  and  to  get  complete  reduction.  If  there  is  no  lacquer 
on  the  wire,  the  electrode  may  be  cleaned  by  heating  with  dilute  nitric  acid 
for  a  short  time.  It  should  be  washed  and  dried  in  exactly  the  same  way 
as  in  the  copper  analysis. 

Caution!  This  treatment  of  the  electrode  should  never  be  given  to  plati- 
num gauze.  The  copper  will  alloy  with  the  platinum  when  heated  in  the 
flame. 


126  QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 

regulate  in  the  same  way  the  rate  at  which  the   liquid   runs 
through  the  siphon. 

As  soon  as  the  solution  shows  but  faint  reaction  with  blue  litmus 
paper,  or  when  lights  in  the  circuit  grow  dim,  it  is  safe  to  break  the 
current  and  to  wash  the  cathode  with  water  and  alcohol  as  de- 
scribed on  page  120. 

This  method  of  removing  the  solution  at  the  end  of  the  elec- 
trolysis is  open  to  the  objection  that  when  other  determinations 
are  to  be  made  after  the  removal  of  the  copper  it  is  often  necessary 
to  evaporate  and  concentrate  the  solution  and  this  causes  a  tedious 
delay.  If  there  is  no  further  use  for  the  liquid  from  which  the 
copper  has  been  removed,  or  if  the  presence  of  acetate  and  acetic 
acid  does  no  harm  to  further  work,  the  nitric  acid  may  be  rendered 
harmless,  after  the  electrolysis  is  over,  by  adding  a  sufficient 
quantity  of  sodium  acetate.  This  salt  reacts  with  the  nitric  acid 
and  forms  free  acetic  acid  which  does  not  exert  an  appreciable 
solvent  effect  upon  the  deposited  copper  (Riidorff). 

The  vessel  shown  in  Fig.  46  is  very  convenient  to  use  when  it 
is  desired  to  wash  a  deposit  before  breaking  the  circuit.  On  filling 
the  vessel  with  solution,  the  latter  comes  to  about 
the  line  a  in  the  siphon  tube  but  during  the  elec- 
trolysis the  bubbles  of  oxygen  from  the  foot  of 
the  anode  cause  enough  diffusion  to  prevent 
this  part  of  the  solution  from  escaping  the  action 
of  the  electric  current.  Similar  vessels  in  which 
a  straight  tube  with  stopcock  is  fused  into  the 
middle  of  the  bottom  of  the  beaker  are  not  so 
satisfactory;  the  solution  flows  down  to  the 
stopcock  and  retains  its  original  density  for  a 
long  time  while  the  solution  above  it  becomes 
specifically  lighter  owing  to  the  removal  of  the 
copper;  thus  the  diffusion  takes  place  very  slowly. 

The  deposition  of  copper  from  a  pure  nitric-acid  solution  is 
advantageous  if  it  is  necessary  to  use  nitric  acid  for  the  solution 
of  the  original  substance  (copper,  its  alloys,  ores,  etc.),  and  if  there 
is  no  reason  to  evaporate  the  solution  with  concentrated  sulphuric 
acid  as  when  it  is  desired  to  remove  the  lead  as  sulphate.  If  such 
an  operation  is  necessary,  it  is  better  to  carry  out  the  analysis  from 
a  pure  sulphuric-acid  solution,  as  described  on  page  116,  without 
the  addition  of  any  nitric  acid.  The  deposition  from  a  nitric-acid 


COPPER  127 

solution  is  to  be  recommended  especially  when  considerable  iron 
is  present  as  in  the  analysis  of  pyrites. 

There  are  two  sources  of  error  to  guard  against  in  the 
electrolysis  by  the  nitric-acid  method.  It  was  stated  on  p.  117 
that  nitric  acid  can  be  reduced  to  ammonia  by  the  action  of 
the  electric  current  during  electrolysis.  If,  therefore,  too  little 
nitric  acid  is  used  there  is  danger  of  the  solution  becoming 
ammoniacal  and  the  metal  will  deposit  in  a  spongy  condition. 
The  formation  of  ammonia  is  disadvantageous  when  it  is  desired 
to  separate  the  copper  from  other  metals  which  are  not  deposited 
while  the  solution  contains  free  nitric  acid.  It  is  always  neces- 
sary, therefore,  to  make  sure  that  the  nitric  acid  in  the  solution 
never  disappears  entirely.  In  this  case  the  deposition  of  the 
metal  can  take  place  at  a  constant  potential  (see  p.  119).  On 
the  other  hand  it  is  not  advisable  to  use  too  much  nitric  acid 
as  this  will  prevent  the  deposition  of  the  copper  until  the  excess 
of  nitric  acid  has  been  reduced  to  ammonia  and  thus  the 
electrolysis  will  require  a  long  time.  Especially  in  hot  solu- 
tions the  retarding  effect  of  an  excess  of  nitric  acid  is  very  pro- 
nounced. 

If  considerable  iron  is  present  in  the  solution  the  ferric  nitrate 
exerts  a  solvent  effect  upon  the  deposited  copper  and  there  are 
thus  two  causes  which  tend  to  retard  the  deposition  of  the 
copper  when  too  much  nitric  acid  is  used.  If  much  iron  is 
present  it  is  necessary  to  limit  the  amount  of  nitric  acid  added 
very  carefully.  For  the  details  of  the  procedure  see  page  293, 
where  the  electrolysis  of  solutions  rich  in  iron  is  described. 

The  conditions  under  which  good  results  have  been  obtained  in 
the  rapid  electrolytic  determination  of  copper  are  given  in  the 
following  table  (p.  128).  As  regards  the  results  obtained  when 
the  electrolyte  is  subjected  to  magnetic  stirring,  see  page  77. 


128  QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 

RAPID  DEPOSITION  OF  COPPER  IN  NITRIC-ACID  SOLUTIONS. 


Experiments  performed  by  A.  Fischer  in  the 

H.  J.  S. 

Aachen  Laboratory. 

Sand* 

Exner. 

Kind  of  elec- 

Platinum dish  and  rotating 

Gauze 

Sand's 

Platinum 

trode. 

disk  electrode. 

electrode 

elec- 

dish and 

and 

trode 

rotating 

lattice 

spiral 

stirrer 

(Fig.  33) 

Electrolyte 

12  to  20 

2  cc. 

1  cc. 

1  cc. 

1  cc. 

1  cc. 

contained. 

cc.  HNO3 

HNO3 

HN03 

HNO3 

HNO3 

HNO3 

(1.2) 

(1.2) 

(1.2) 

(1.4) 

(1.4) 

(1.4) 

+  5cc. 

NH4OH 

(0.96) 

after  10 

minutes 

Volume 

125  cc. 

125  cc. 

125  cc. 

110  cc. 

85  cc. 

125  cc. 

Quantity  of 

0.3  gm. 

0.3  gm. 

0.3  gm. 

0.3  gm. 

0.24  gm. 

Ot.24-0.29 

metal. 

as  sul- 

as sul- 

as sul- 

as sul- 

as sul- 

* gm. 

phate 

phate 

phate 

phate 

phate 

Temperature  .  .  . 

20°-30° 

95° 

90° 

Hot 

Hot 

90° 

Voltage  of  the 

2.5  to  3 

3  to  3.5 

8.5  volts 

2.8  to  3 

2.8  volts 

8-10 

bath. 

volts 

volts 

volts 

volts 

Current 

10  amp. 

strength. 

Number  of  rev- 

800 to 

800 

800 

1000  to 

800 

800 

olutions. 

1000 

1200 

Duration  in 

52  to  62 

40 

20 

10 

6 

15  to  20 

minutes. 

*  J.  Chem.  Soc.,  London,  91,  391  (1907). 

Rapid  Electrolysis  with  Stationary  Electrolyte.  With  the  aid 
of  a  gauze  cathode  it  is  possible  to  use  enough  current  to  get 
0.5  gm.  of  copper  deposited  in  less  than  fifteen  minutes,  even 
without  stirring  the  electrolyte.  There  is,  however,  much  more 
danger  of  getting  spongy  deposits  than  when  the  electrolyte  is 
stirred.  The  size  of  the  electrode,  volume,  and  acid-content  of 
the  solution  are  factors  which  must  be  kept  within  narrow  limits. 
The  following  procedure,  if  followed  closely,  has  been  found  to 
give  good  results  in  the  analysis  of  brass. 

Procedure.  Dissolve  0.5  gm.  of  the  metal  in  10  cc.  of  6-normal 
nitric  acid.  Dilute  to  50  cc.  in  a  tall,  slender,  lipless  beaker 
of  about  80  to  102-cc.  capacity,  Cover  the  beaker  and  boil  very 


COPPER  129 

gently  for  about  one  minute  to  remove  nitrous  fumes.  Add 
6-normal  ammonium  hydroxide  slowly  until  a  slight  permanent 
precipitate  is  formed  and  then  add  enough  6-normal  sulphuric 
acid  (about  0.5  cc.)  to  cause  this  precipitate  to  dissolve.  Elec- 
trolyze  this  solution  with  a  current  of  about  1  ampere  until  all 
the  copper  is  deposited.  As  cathode  use  a  platinum  gauze  cyl- 
inder 3  cm.  or  more  long  and  about  3  cm.  in  diameter  with  ap- 
proximately 20  meshes  to  the  linear  centimeter.  As  anode  a 
platinum  spiral  may  be  used  and  it  should  be  placed  in  the  center 
of  the  cylinder.  Cover  the  beaker  with  split  watch  glasses,  to 
prevent  loss,  and  keep  the  solution  heated  to  about  70°  during 
the  electrolysis.  When  the  solution  has  become  colorless,  add 
about  0.2  gm.  of  urea,  wash  down  the  sides  of  the  beaker  and 
bottom  of  the  cover  glass,  and  continue  the  electrolysis  a  little 
longer.  It  is  important  not  to  continue  the  electrolysis  much 
longer  than  necessary  to  remove  all  the  copper.  At  the  most, 
one  hour  should  be  sufficient.  Test  the  solution  for  copper,  as 
directed  on  page  120,  and  finish  the  work  as  there  described. 

Deposition  of  Copper  from  Ammoniacal  Solutions. 

When  either  of  the  above  methods  is  used  for  the  electrolytic 
determination  of  copper,  the  solution  should  not  contain  any 
chloride  as  the  latter  usually  gives  rise  to  a  spongy  deposit  of  cop- 
per and,  moreover,  there  is  danger  of  the  platinum  anode  being 
attacked;  the  dissolved  platinum  will  then  deposit  upon  the 
cathode.  If  a  solution  of  a  copper  salt  contains  chloride,  and  it  is 
desired  to  avoid  evaporation  with  .sulphuric  acid,  the  electrolytic 
determination  may  be  carried  out  in  an  ammoniacal  solution. 

This  method  also  possesses  certain  advantages  over  other  meth- 
ods when  it  is  desired  to  effect  the  separation  from  a  metal  such  as 
antimony.  Riidorff  obtained  a  compact  deposit  of  copper  from  an 
ammoniacal  solution  to  which  ammonium  nitrate  was  added. 

In  the  laboratory  of  the  Munich  Polytechnic  Institute,  the 
following  directions  have  been  worked  out  for  the  electrolysis  in 
ammoniacal  solution.  Add  ammonia  to  the  copper  solution 
(chloride,  nitrate  or  sulphate)  in  slight  excess,  or  until  the  precipi- 
tate formed  redissolves.  Then,  if  not  more  than  0.5  gm.  of  copper 
is  present,  add  20  to  25  cc.  more  of  ammonia,  sp.  gr.  0.96.  If  as 
much  as  1  gm.  of  copper  is  present,  increase  the  quantity  of 


130  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

ammonia  added  to  30  or  35  cc.  Dissolve  2  or  3  gms.  of  ammonium 
nitrate  in  this  solution  and  electrolyze  with  a  current  NDioo  =  2 
amperes.  Wash  the  deposit  before  breaking  the  circuit. 

The  presence  of  chlorine,  zinc,  arsenic  and  small  amounts  of 
antimony  do  no  harm  when  this  method  is  followed;  in  the  pres- 
ence of  lead,  bismuth,  mercury,  cadmium  and  nickel,  the  results 
are  too  high. 

In  sulphuric-  or  nitric-acid  solutions  the  copper  is  present  largely 
in  the  form  of  simple,  bivalent  copper  ions,  Cu++,  and  when  the 
concentration  of  the  ions  is  diminished  as  a  result  of  their  discharge 
at  the  cathode,  then  the  undissociated  molecules  of  CuSCU  or 
Cu(N03)2  quickly  dissociate  to  form  new  cupric  ions.  In  ammo- 
niacal  solutions,  of  the  sulphate  for  example,  the  complex  salt 
[Cu(NH3)4]S04  is  formed,  which  dissociates  first  into  the  complex 
cupric  ammonia  cation  [Cu(NH3)4]++  and  SO^  anions.  The  cupric 
ammonia  ions  are  not  very  stable  and  break  down  to  an  appreci- 
able degree  as  illustrated  by  the  equilibrium  expression, 

[Cu(NH3)4]++  <=±  Cu++  +  4  NH3, 

and  the  concentration  of  cupric  ions  resulting  from  such  dissocia- 
tion is  sufficient  to  permit  the  deposition  of  copper  when  the 
potential  of  the  current  is  less  than  2  volts.  For,  according  to 
the  formula  on  page  26,  the  cathode  potential  depends  upon  the 
osmotic  pressure  and  thus  upon  the  concentration  of  the  metal 
ions,  and  as  long  as  this  potential  is  less  than  the  potential  between 
the  electrodes  of  the  cell,  there  will  be  deposition  of  metal. 

According  to  Foerster's  experiments,  it  is  possible  to  effect  the 
electrolytic  deposition  of  copper  from  ammoniacal  solutions  by  the 
use  of  a  single  lead  accumulator  cell,  as  in  the  electrolysis  of  sul- 
phuric-acid solutions  of  copper.  Foerster  takes  the  solution  con- 
taining 0.2  to  0.3  gm.  copper  in  100  cc.,  adds  2  gms.  ammonium 
sulphate  and  10  cc.  ammonia  (sp.  gr.  0.96)  and  obtains  a  quanti- 
tative deposition  in  4  hours  with  one  accumulator  cell.  Under 
these  conditions  the  copper  is  separated  from  arsenic  if  the  quan- 
tity of  the  latter  present  in  100  cc.  of  the  solution  is  not  more 
than  0.2  gm.;  but  it  is  absolutely  necessary  that  all  the  arsenic 
be  present  as  arsenate  (see  page  233) . 


SILVER  131 

Silver. 

At.Wt.  =  107.88.  Elec.Equiv.  =  1.118  mg.  Elec.  Potential  = 
—  0.771  volt  for  Ag  +  ions.  Overvoltage  of  H2  =  between 
0.05-0.15  volt. 

Of  the  various  methods  for  the  electrolytic  determination  of 
silver  only  those  using  nitric  acid,  potassium  cyanide  and  am- 
monium hydroxide  solutions  as  electrolytes  will  be  considered. 

Deposition  of  Silver  from  Nitric-acid  Solution. 

According  to  the  studies  of  F.  W.  Kiister  and  H.  von  Steinwehr,* 
the  electrolytic  determination  of  silver  succeeds  best  if  the  solu- 
tion, which  may  contain  from  0.3  to  2  gms.  of  silver  in  150  cc., 
is  heated  to  55°  or  60°,  treated  with  1  or  2  cc.  of  nitric  acid  f  (sp. 
gr.  1.4)  and  5  cc.  of  alcohol,  and  electrolyzed  with  the  potential 
of  the  bath  kept  constant  between  1.35  and  1.38  volts.  The 
deposited  metal  must  be  washed  without  breaking  the  current  and 
dried  at  about  100°.  As  cathode,  a  platinum  dish  with  dull  inner 
surface  and  as  anode  a  disk  or  spiral  may  be  used. 

The  addition  of  the  alcohol  serves  to  reduce  immediately  any 
silver  peroxide  that  may  be  formed  during  the  process. 

According  to  the  above-mentioned  authors,  the  most  important 
condition  for  a  successful  electrolysis  is  keeping  the  voltage  con- 
stant within  the  stated  limits.  If  the  voltage  rises  above  1.38 
volts  a  spongy  deposit  is  obtained.  The  unreliability  of  most 
other  methods  for  the  electrolysis  of  a  nitric-acid  solution  of  silver 
salt  can  be  traced  to  the  use  of  too  high  voltages.  In  the  older 
methods,  chief  stress  was  laid  upon  the  current  density,  so  that 
although  the  potential  was  right  at  the  start  of  the  analysis,  dur- 
ing the  progress  of  the  electrolysis  it  rose  above  the  critical  value 
as  the  solution  became  deprived  of  metal  ions  (cf.  p.  89). 

It  is,  therefore,  very  important  in  the  electrolytic  determina- 
tion of  silver  to  use  a  source  of  current  such  that  the  voltage 
cannot  rise  above  1.38  volts.  For  this  purpose  a  Giilcher's 
thermopile  may  be  used  which  has  a  maximum  voltage  of  about 
4  volts.  If  a  wire  resistance  of  suitable  length  is  inserted  between 

*Z.  Elektrochem.,  4,  451  (1898). 

t  If  from  0.3  to  2  gms.  of  a  silver  alloy  is  dissolved  in  2  to  4  cc.  of  nitric 
acid  (sp.  gr.  1.4)  the  further  addition  of  acid  is  unnecessary. 


132  QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 

the  binding  posts  of  the  thermopile,  the  current  can  be  adjusted 
so  that  the  electromotive  force  of  the  current  from  the  pile  is 
reduced  to  1.36  volts  and  it  is  then  only  necessary  to  connect  the 
electrolytic  cell  directly  with  these  binding  posts.  Instead  of 
short  circuiting  the  terminals  with  a  resistance  wire,  another  way 
of  getting  the  proper  voltage  is  to  connect  one  electrode  of  the  cell 
with  the  binding  post  and  the  other  electrode  with  one  of  the 
metal  wings  of  the  thermopile.  It  is  possible,  however,  to  get  a 
finer  adjustment  of  the  voltage  by  the  use  of  resistance  wire. 

In  accordance  with  wha.1  was  said  on  page  118  concerning  work 
carried  out  at  a  constant  voltage,  the  strength  of  the  current  will 
necessarily  diminish  constantly  during  the  progress  of  the  elec- 
trolysis and  thus  a  determination  will  require  from  six  to  eight 
hours.  The  quantity  of  metal  present  in  the  solution  has  but 
little  influence  upon  the  duration  of  the  analysis  because  the 
strength  of  the  current  is  greater  in  proportion  to  the  concentration 
of  the  silver  solution.  Thus  it  is  the  deposition  of  the  last  traces 
of  metal  which  requires  the  most  time  and  this  is  about  the  same 
in  all  cases. 

Rapid  Deposition  of  Silver  from  Nitric-acid  Solution. 

Two  difficulties  often  encountered  in  the  rapid  electrolysis  of 
silver  solutions  are  the  formation  of  large  crystals  on  the  cathode 
and  the  deposition  of  a  little  silver  peroxide  on  the  anode.  By 
stirring  the  electrolyte,  keeping  the  solution  hot,  and  control- 
ling the  cathode  potential,  these  objectionable  features  can  be 
avoided.* 

Procedure.  To  about  85  cc.  of  the  neutral  solution  containing 
up  to  0.5  gm.  of  silver  as  nitrate,  add  2  to  5  cc.  of  5-normal  nitric 
acid  and  electrolyze  at  100°  with  a  platinum  gauze  cathode  and 
a  rotating  spiral  anode.  Begin  with  a  current  of  about  3.5 
amperes  with  1.5  volts  e.m.f.  between  the  terminals  and  cause  the 
anode  to  revolve  at  the  rate  of  800  to  1000  r.p.m.  With  the 
aid  of  the  apparatus  described  on  page  148,  keep  the  cathode 
potential  to  0.1  volt  or  less,  so  that  at  the  end  of  fifteen  minutes, 
when  the  electrolysis  should  be  finished,  the  current  will  be  re- 
duced to  0.2  ampere.  Wash,  dry  and  weigh  the  deposit  as 
described  under  Copper. 

*  C/.  Fischer,  Elektroanalytische  Schnellmethoden,  Stuttgart,  1908. 


SILVER  133 

Deposition  of  Silver  from  Ammoniacal  Solution. 

Silver  when  deposited  from  ammoniacal  solutions  with  a 
stationary  electrolyte  is  likely  to  be  in  the  form  of  a  spongy  de- 
posit, not  suitable  for  accurate  weighing.  According  to  Sand  * 
dense  deposits  can  be  obtained  which  cannot  be  rubbed  off  if 
the  following  procedure  is  followed: 

Procedure.  To  the  neutral  solution  containing  about  0.5  gm. 
of  silver  as  nitrate,  add  10  cc.  of  15-normal  nitric  acid  and  25  cc. 
of  15-normal  ammonium  hydroxide.  With  a  total  volume  of 
85  cc.,  and  using  a  gauze  cathode  and  a  platinum  spiral  anode 
revolving  800  r.p.m.,  start  the  electrolysis  with  a  current  of  4 
amperes  and  1  to  1.3  volts  between  the  terminals.  During  the 
progress  of  the  electrolysis  do  not  let  the  voltage  rise  higher  than 
this  value,  so  that  at  the  end  of  about  ten  minutes,  when  all 
the  metal  should  be  deposited,  the  current  will  be  reduced  to  about 
0.2  ampere. 

The  use  of  the  ammoniacal  electrolyte  is  particularly  suitable 
when  silver  is  to  be  deposited  in  the  presence  of  arsenic  and 
antimony. 

Deposition  of  Silver  from  Potassium-cyanide  Solution. 

Luckow  first  suggested  the  determination  of  silver  by  the 
electrolysis  of  the  complex  silver-potassium  cyanide  (cf.  p.  51). 
If  a  neutral  silver  solution  is  at  hand,  add  potassium  cyanide 
solution  until  the  silver  cyanide  precipitate  redissolves  and  then 
add  as  much  more  of  the  cyanide  solution.  Dilute  the  solution 
to  a  volume  of  100  to  120  cc.  and  carry  out  the  electrolysis  with 
a  current  of  NDioo  =  0.2  to  0.5  ampere.  The  potential  of  the 
bath  under  these  conditions  lies  between  3.7  and  4.8  volts.  With 
the  same  quantity  of  silver,  the  electrolysis  requires  from  5  to 
1.5  hours,  according  to  whether  0.2  or  0.5  ampere  of  current 
is  used.  The  temperature  of  the  solution  should  be  between 
20°  and  30°.  To  determine  whether  the  deposition  of  metal 
is  complete,  add  nitric  acid  to  a  little  of  the  solution,  boil  off  the 
hydrogen  cyanide  under  a  good  hood  and  test  for  silver  with 
ammonia  and  ammonium  sulphide.  If  a  black  precipitate  of 
silver  sulphide  is  obtained;  it  should  be  filtered  off,  dissolved  in 

*  Proc.  Chem.  Soc.,  22,  43  (1906). 


134          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

nitric  acid,  the  solution  treated  with  potassium  hydroxide  solu- 
tion till  alkaline,  then  with  potassium  cyanide  and  the  resulting 
solution  added  to  the  original  electrolyte  for  further  electrolysis. 

If  insoluble  silver  compounds,  such  as  the  chloride,  bromide, 
iodide  or  oxalate,  are  to  be  analyzed,  they  are  dissolved  in 
potassium-cyanide  solution. 

This  was  the  only  reliable  method  for  determining  silver  electro- 
lytically  until  the  method  of  Kuster  and  v.  Steinwehr  was  pub- 
lished. It  is  important  to  use  pure  potassium  cyanide  in  the 
analysis  as  the  presence  of  small  quantities  of  cyanate  or  other 
impurity  prevents  the  adherence  of  the  silver  to  the  cathode. 

Rapid  Deposition  of  Silver  from  Cyanide  Solution. 

In  spite  of  the  tendency  to  get  a  little  silver  peroxide  formed 
on  the  anode,  Gooch  and  Medway  *  and  E.  F.  Smith  f  have 
obtained  good  results  in  the  rapid  electrolysis  of  silver  Jn  alkali 
cyanide  solutions.  Gooch  and  Medway  used  as  cathode  a  plat- 
inum crucible  which,  with  the  aid  of  a  rubber  stopper,  was  fas- 
tened to  the  end  of  a  rotating  metal  shaft  and  was  connected 
also  with  the  negative  pole  of  the  electrolytic  circuit.  Smith, 
on  the  other  hand,  rotated  the  anode,  and  kept  the  solution 
near  the  boiling  point. 

Procedure.  To  the  neutral  solution  of  silver  nitrate  containing 
as  much  as  0.5  gm.  of  silver,  add  2  gms.  of  potassium  cyanide. 
Dilute  to  about  125  cc.,  heat  nearly  to  boiling  and  electrolyze 
the  hot  solution  with  a  current  of  NDioo  =  2  to  2.8  amperes 
and  rotating  the  anode  700  r.p.m.  A  platinum  gauze  electrode 
may  serve  as  cathode.  All  of  the  silver  will  be  deposited  in  about 
ten  minutes. 

*  Am.  J.  Sci.,  4,  15,  320. 

t  Electro-analysis,  1918,  p.  117. 


MERCURY  135 

Mercury. 

At.  Wt.  =  200.6.  Elec.  Equiv.  =  2.078  for  Hg  +  +ions.  Over- 
voltage  of  H2  =  0.42-0.78  volt.  Elec.  Potential  =  -  0.750 
volt. 

Deposition  from  Nitric-acid  Solution. 

The  solution  containing  the  metal  as  nitrate,  chloride  or  sul- 
phate is  treated  with  1  or  2  per  cent  by  volume  of  nitric  acid 
(sp.  gr.  1.36)  and  electrolyzed  at  room  temperature  with  a  current 
of  NDioo  =  1.0  ampere. 

The  solution  may  contain  small  quantities  of  hydrochloric  acid, 
or  chloride,  but  large  quantities  are  harmful.  If  other  metals 
are  present  which  require  acid  to  prevent  their  precipitation,  5  per 
cent  by  volume  of  nitric  acid  should  be  added  and  the  current 
density  reduced  to  0.5  ampere. 

A  roughened  platinum  dish  or  a  gauze  electrode  must  be  used 
as  cathode.  The  mercury  deposits  upon  these  electrodes  as  a  uni- 
form coating,  whereas  if  a  polished  electrode  is  used  it  is  obtained 
in  the  form  of  small  globules.  To  test  whether  the  deposition  is 
complete,  a  little  of  the  solution  may  be  treated  with  ammonia 
and  ammonium  sulphide,  or  a  bright  copper  or  gold  wire  may  be 
suspended  in  the  solution  over  the  cathode  and  watched  to  see 
whether  it  becomes  amalgamated.  In  all  cases,  the  deposit  must 
be  washed  without  interrupting  the  current  and  only  water 
should  be  used,  because  alcohol  tends  to  loosen  the  film  of  mer- 
cury from  the  cathode.  On  account  of  the  volatility  of  this  metal, 
the  electrode  must  be  dried  at  the  room  temperature  in  a  desiccator. 

To  avoid  slight  losses  which  may  result  even  then,  Borelli  * 
recommends  that  a  dish  of  mercury  be  placed  in  the  bottom  of 
the  desiccator  so  that  the  air  there  is  kept  saturated  with  mer- 
cury vapors. 

Again,  owing  to  the  volatility  of  mercury,  it  is  not  advisable 
to  carry  out  the  analysis  from  a  heated  electrolyte,  for,  if  the 
electrolysis  is  carried  out  for  a  long  time,  some  of  the  liquid  will 
evaporate  and,  unless  the  cell  is  closely  watched,  this  will  leave 
an  exposed  mercury  surface  from  which,  if  hot,  appreciable 
volatilization  of  mercury  may  take  place.  The  dish,  or  beaker, 
should  be  kept  covered  with  a  watch  glass  during  the  analysis. 
*  Z.  Elektrochem.,  12,  889  (1906). 


136 


QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS 


The  following  table  gives  the  conditions  under  which  the  rapid 
electrodeposition  of  mercury  has  been  obtained  successfully.* 

RAPID  DEPOSITION  OF  MERCURY  FROM  NITRIC-ACID  SOLUTION. 


Experiments  performed  by 

A.  Fischer 
and  Boddaert 
at  Aachen. 

Exner. 

R.  O.  Smith. 

H.  J.  S.  Sand. 

Electrode  

Dish  and 
rotating 
disk 

Dish  and 
rotating 
spiral 

Dish  and 
rotating 
spiral 

Sand's  elec- 
trodes 

Electrolyte  contained  . 

1  cc.  HN03 

(sp.  gr.  1.4) 

1  cc.  HNO3 

(sp.  gr.  1.4) 

1  cc.  HNO3 

(sp.  gr.  1.4) 

1.5  cc. 
HNO3 

Volume  

125  cc. 

125  cc. 

115  cc. 

85  cc. 

Quantity  of  metal  

0.23  gm.  as 
chloride 

0.3  to  0.6 
gm.  as 
nitrate 

0.25  to  0.5 
gm.  as 
nitrate 

0.58  gm.  as 
nitrate 

Temperature  t  

22°  to  45° 

Hot 

Hot 

Warm 

Deposition  of  Mercury  from  Potassium-cyanide  Solution. 

This  method,  proposed  by  Edgar  F.  Smith,  gives  good  results 
when  carried  out  as  follows:  To  the  solution,  containing  not  more 
than  0.5  gm.  of  mercuric  chloride,  add  3  gms.  of  potassium  cyan- 
ide, whereby  a  clear  solution  of  complex  potassium-mercuric 
cyanide,  K2Hg(CN)4,  is  obtained  in  which  the  mercury  is  present 
as  the  bivalent  mercuric-cyanide  anion.  After  diluting  the  solu- 
tion to  150  cc.,  carry  out  the  electrolysis  at  room  temperature 
with  a  current  of  from  0.5  to  1  ampere;  the  analysis  is  finished 
in  about  15  hours.  To  determine  whether  the  deposition  is 
complete,  take  out  a  little  of  the  solution  with  a  pipette,  add 
nitric  acid,  boil  off  the  hydrogen  cyanide,  and  test  for  mercury  with 
ammonia  and  ammonium  sulphide.  If  a  negative  test  for  mercury 
is  obtained  finish  the  electrolysis  as  in  the  previous  method. 

Higher  current  densities  are  to  be  avoided  because  these  will 
heat  the  solution  which  is  likely  to  cause  volatilization  of  some 

*  General  reference  to  the  literature  on  the  rapid  electrolytic  deposition  and 
separation  of  mercury  in  different  solutions:  Exner,  J.  Am.  Chem.  Soc.,  25,  896 
(1903) ;  A.  Fischer  and  Boddaert,  Z.  Elektrochem,  10,  945  (1904) ;  R.  O.  Smith, 
Thesis  U.  of  P.,  1905;  A.  Fischer,  Chem.-Ztg.,  31,  25  (1907);  E.  F.  Smith, 
and  Kollock,  J.  Am.  Chem.  Soc.,  27,  1527  (1905). 

t  Regarding  the  effect  of  temperature,  see  the  above  text. 


MERCURY  137 

of  the  mercury.  Moreover,  the  platinum  electrode  is  attacked 
by  a  hot  solution  of  potassium  cyanide.  There  are  thus  two 
sources  of  error  if  the  electrolysis  is  carried  out  in  a  hot  solution. 

The  rapid  electrodeposition  of  mercury  from  a  cyanide  solution 
is  inexpedient  for  the  same  reason,  as  such  methods  involve  higher 
current  densities  or  hot  solutions. 

Insoluble  mercury  compounds,  as  mercuric  sulphide  or  mercur- 
ous  chloride,  are  suspended  in  a  solution  of  common  salt,  or  in 
very  dilute  hydrochloric  acid,  and  electrolyzed  under  current 
conditions  described  for  the  electrolysis  of  nitric-acid  solutions. 
(See  also  the  article  on  Cinnabar.) 

Deposition  of  Mercury  from  Sodium  Sulphide  Solution 

E.  F.  Smith  *  has  found  that  an  alkaline  sulphide  solution  of 
mercuric  salt  can  be  electrolyzed  without  difficulty. 

Procedure.  Add  20  cc.  of  sodium  sulphide  solution,  sp.  gr. 
1.19,  to  the  neutral  solution  of  the  mercuric  salt  and  dilute  with 
water  to  a  volume  of  125  cc.  Electrolyze  in  a  platinum  dish, 
which  also  serves  as  cathode,  with  a  platinum  spiral  as  anode. 
Use  a  current  of  NDioo  =  0.11  ampere  at  70°  for  five  hours.  Keep 
the  dish  covered  during  the  electrolysis  to  prevent  evaporation, 
to  avoid  mechanical  loss,  and  to  prevent  mercuric  sulphide  being 
formed  at  the  top  of  the  deposit  where  the  solution  has  evap- 
orated away. 

When  the  electrolysis  is  finished,  siphon  off  the  solution,  and 
wash  the  deposit  with  cold  water:  Dry  on  a  moderately  warm 
plate  or  in  a  desiccator  over  sulphuric  acid. 

*  Electro-analysis,  1918,  p.  101. 


138  QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 

Gold. 

At.  Wt.  =  197.2.  Elec.  Equiv.  =  0.328  mg.  for  AU  +  +  +  ions. 
Elec.  Potential  =  <  -  1.083  volt.  Overvoltage  of  H2  = 
0.02-0.06  volt. 

When  deposited  from  acid  or  alkaline  solutions,  brown,  non- 
adherent,  amorphous  gold  is  often  formed  and,  in  many  cases, 
the  electrolyte  itself  is  of  a  greenish  or  purplish  hue,  due  to  gold 
remaining  in  colloidal  solution.  Satisfactory  deposits,  however, 
may  be  obtained:  (1)  from  a  solution  in  potassium  cyanide, 
(2)  from  a  solution  in  sodium  sulphide,  and  (3)  from  a  solution 
in  ammonium  thiocyanate. 

Deposition  from  Potassium-cyanide  Solution. 

If  a  slightly  acid  solution  of  auric  chloride  is  treated  with  a 
solution  of  potassium  cyanide,  a  yellow  precipitate  of  auric 
cyanide  is  obtained  which  dissolves  in  an  excess  of  potassium 
cyanide  forming  colorless  potassium  auricyanide: 

AuCl3  +3  KCN  =  Au(CN)3  +  3  KC1 
Au(CN)3+    KCN=KAu(CN)4. 

This  salt  dissociates  into  K+  and  Au(CN)T  ions;  the  gold,  there- 
fore is  a  constituent  of  the  anion,  which  is  itself  dissociated  to  a 
slight  extent : 

Au(CN)r«=*Au++++  +4CN~; 

and  as  the  gold  ions  are  discharged  at  the  cathode  more  of  them 
are  formed  by  the  progressive  dissociation  of  the  complex  anion. 
To  prepare  a  suitable  electrolyte,  the  gold  solution,  which  should 
not  contain  too  much  free  acid,*  is1  treated  with  2  or  3  gms.  of 
pure  potassium  cyanide,  and  diluted  to  120  cc.  The  electrolysis 
is  conducted  in  the  solution  heated  to  60°  in  a  roughened  platinum 
dish,  using  a  current  of  NDioo  =  2.7  to  4  volts.  The  deposition 
of  0.05  gm.  of  gold  requires  2  or  3  hours.  If  the  electrolysis  is 
carried  out  at  ordinary  temperatures,  the  complete  deposition  of 
the  same  quantity  of  gold  requires  12  to  14  hours. 

*  If  much  acid  is  present,  it  is  removed  either  by  evaporation  at  a  temper- 
ature too  low  to  cause  decomposition  of  the  auric  chloride,  or  by  neutraliza- 
tion with  caustic  potash  solution.  For  a  practice  experiment,  crystallized 
sodium  chloraurate,  NaAuCl4  •  2H2O,  may  be  used  or  pure  gold  may  be  dis 
solved  in  aqua  regia  and  the  excess  of  acid  removed  by  evaporation. 


GOLD 


139 


The  end  of  the  electrolysis  is  determined,  as  described  on  page 
125,  by  raising  the  level  of  the  solution. 

The  following  table  shows  the  conditions  under  which  gold  has 
been  determined  rapidly  from  well-stirred  solutions. 

RAPID  DEPOSITION  OF  GOLD  FROM  POTASSIUM-CYANIDE 

SOLUTION.* 


Experiments  performed  by 

Withrow;  Exner. 

A.  Fischer. 

H.  E.  Medway. 

Electrode  

Dish  and  ro- 
tating spiral 

1  to  2  gms. 
KCN 

80  to  125  cc. 

0.14  to  0.2  gm. 
as  AuCl3 

Boiling 
11  to  10.5  volts 
800  to  500 
6  to  10 

Dish  and  ro- 
tating disk 

1  to  2  gms. 
KCN 

100  cc. 

0.1  to  0.15  gm. 
as  AuCl3 

Boiling 
8  to  10  volts 
800 
10 

Rotating  cru- 
cible cathode 

Excess  of 
KCN,  40.1  cc. 
cone.  NH4OH 

25  cc. 

0.065  gm.  as 
AuCl3 

Ordinary 
t 
650  to  700 
25  to  30 

Electrolyte  contained  
Volume 

Quantity  of  metal 

Temperature  

Voltage  

Revolutions  

Duration  in  minutes  

Deposition  of  Gold  from  Sodium-sulphide  Solution.! 

Gold  solutions  behave  toward  alkali-sulphide  solutions  similar 
to  those  of  antimony.  If  a  gold-chloride  solution  is  treated  with 
sodium-sulphide  solution,  a  brownish  precipitate  of  gold  sulphide 
is  formed  which  dissolves  upon  the  addition  of  a  considerable 
excess  of  the  reagent  forming  sodium  thioaurate.  The  decom- 
position of  this  salt  by  the  electric  current  takes  place  as  in  the 
electrolysis  of  the  corresponding  antimony  solution  (p.  154), 
and  the  deposition  of  the  gold  is  a  result  of  a  purely  secondary 
reaction. 

The  gold  solution  is  treated  with  a  sufficient  amount  of  sodium- 

*  General  references  on  the  rapid  deposition  and  separation  of  gold  from 
various  solutions:  Medway,  Am.  J.  Sci.  [4],  18,  56  (1904),  Z.  anorg.  Chem., 
42,  114  (1904).  Exner,  J.  Am.  Chem.  Soc.,  25,  896  (1903).  Withrow, 
Thesis,  U.  of  P.,  1905.  E.  F.  Smith  and  Kollock,  J.  Am.  Chem.  Soc.,  27, 
1527  (1905). 

f  The  voltage  is  not  given  but  the  current  density  was  ND10o  =  1.8  to  3.3 
amperes. 

|  Smith  and  Wallace,  Ber.,  25,  779  (1892). 


140  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

sulphide  solution,  saturated  at  room  temperature,  to  cause  the 
complete  solution  of  the  gold  precipitate  that  is  first  formed,  and 
the  resulting  solution  is  electrolyzed  with  a  current  of  NDi00  = 
0.1  to  0.25  ampere  for  5  or  6  hours. 

Deposition  of  Gold  from  the  Solution  in  Ammonium 
Thiocyanate. 

F.  M.  Perkin  and  W.  C.  Preble  *  have  found  that  gold  can  be 
deposited  equally  well  from  a  solution  in  ammonium  thiocyanate. 
The  gold  solution  is  poured,  with  constant  stirring,  into  a  warm 
solution  (50°  to  60°)  of  70  or  80  gms.  NH4CNS  in  70  or  80  cc.  of 
water.  After  diluting  with  water  to  a  volume  of  120  cc.,  the 
solution,  which  is  reddish  at  first  but  later  becomes  colorless,  is 
electrolyzed  with  a  current  of  NDioo  =  0.2  to  0.4  ampere  either 
at  the  laboratory  temperature  or  at  a  temperature  of  40°  to  50°. 
In  the  former  case  the  time  required  is  4  to  6  hours ;  in  the  latter, 
1.5  to  2  hours. 

Although  in  the  other  two  methods  the  gold  deposit  has  a  pure 
yellow  color,  by  this  method  a  darker  deposit  is  obtained  some- 
times with  equally  accurate  results.!  If,  however,  potassium 
thiocyanate  is  used  as  solvent  instead  of  the  ammonium  salt,  a 
discolored  deposit  is  obtained. 

To  determine  whether  all  the  gold  has  been  deposited,  the  solu- 
tion is  tested  as  described  on  page  125,  or  a  little  of  it  is  boiled 
with  a  few  drops  of  concentrated  sulphuric  acid  and  a  little  stan- 
nous-chloride  solution  is  added;  if  gold  is  present  the  purple-of- 
Cassius  test  is  obtained. 

Various  suggestions  have  been  made  with  regard  to  the  removal 
of  the  gold  from  the  platinum  electrode  but  the  simplest  method, 
according  to  Perkin  and  Preble,  is  the  treatment  with  a  potassium- 
cyanide  solution  to  which  3  or  4  cc.  of  hydrogen  peroxide  or  a 
little  ammonium  persulphate  is  added.  The  gold  will  dissolve 
in  a  few  seconds. 

*  Electrochemist  and  Metallurgist,  3,  490  (1904). 

t  A  yellow  precipitate  is  often  noticed  in  the  solution;  this  is  "Kanarin," 
a  dyestuff  formed  by  the  anodic  oxidation  of  the  thiocyanate. 


PLATINUM  141 


Platinum. 

At.  Wt.  =  195.2.  Elec.  Equiv.  =  0.505  mg.  for  Pt  +  +  +  +  ions. 
Elec.  Potential  =  <  -0.863  volt.  Overvoltage  for  H2  =0.07- 
0.09  volt. 

Although  it  is  difficult  to  get  a  gold  deposit  that  will  adhere 
to  the  electrode  in  an  acid  solution,  in  the  case  of  an  acid  platinum 
solution  it  is  easy  to  obtain  a  deposit  which  will  adhere  to  either  a 
polished  or  a  roughened  platinum  surface. 

If  the  platinum,  as  is  usually  the  case,  is  present  as  chloro- 
platinic  acid,  H2PtCl6,  the  solution  is  acidified  with  2  per  cent  by 
volume  of  dilute  sulphuric  acid  (1  : 5)  heated  to  60°  or  65°  and 
electrolyzed  with  a  current  of  NDioo  =  0.01  to  0.05  ampere. 
The  potential,  which  is  about  1.2  volts  at  the  start,  rises  later  to 
1.7  volts  and  as  much  as  0.4  gm.  of  the  metal  is  deposited  quan- 
titatively in  5  hours.  The  determination  is  so  accurate  that 
W.  Halberstadt  has  used  it  for  the  determination  of  the  atomic 
weight  of  platinum.  When  all  the  platinum  has  been  deposited, 
a  little  of  the  solution,  on  being  heated  with  hydrogen-sulphide 
water,  will  not  show  a  brown  coloration. 

After  breaking  the  circuit  the  precipitate  can  be  washed  and  it 
adheres  so  well  that  there  is  no  need  to  remove  it  at  the  end  of  the 
analysis.  After  polishing  with  sea  .sand,  the  dish  is  again  ready 
for  use.* 

By  stronger  currents  (0.1  to  0.2  ampere)  the  platinum  is  de- 
posited at  ordinary  temperatures  in  the  form  of  platinum  black. 
That  the  metal  in  this  state  is  used  for  the  preparation  of  plati- 
nized electrodes  was  mentioned  on  page  82. 

The  electrolytic  determination  of  platinum  may  be  used  for 
the  quantitative  estimation  of  potassium  and  sodium  (see  these 
metals) . 

According  to  Julia  Langness  it  is  possible  to  deposit  platinum 
rapidly  under  the  following  conditions. 

According  to  the  experience  of  A.  Fischer  in  the  author's  labo- 
ratory, the  method  is  not  to  be  recommended. 

*  If  it  is  desired  to  remove  the  deposit,  the  electrode  should  be  given 
a  preliminary  coating  of  copper  or  silver  (pp.  172, 173).  Then  on  heating  with 
nitric  acid  the  deposits  will  be  loosened. 

t  J.  Am.  Chem.  Soc.,  29,  459  (1907). 


142 


QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 


RAPID   DEPOSITION   OF  PLATINUM   FROM   SULPHURIC-ACID 

SOLUTION. 


Experiments  of  J.  Langness.f 

Electrode  

Silvered  dish  and  sieve 
anode 

2.5  to  5  cc.  H2SO4  (1  :  10) 
60  cc. 
0.1  gm.  asK2PtC!6 
Hot 
5  to  10  volts 
10  to  14  amp. 
600 
3  to  7 

Silvered  dish  and  sieve 
anode 

2.5  cc.  H2SO4  (1  :  10) 
60  cc. 
0.2  gm.  as  K2PtCl6 
Hot 
10  volts 
17  amp. 
600 
5 

Electrolyte  .            »*•'•• 

Volume           

Quantity  of  met  si 

Temperature 

Potential                  

Current  strength  
Revolutions 

Time  in  minutes 

Palladium. 

At.  Wt.  =  106.7.  Elec.  Equiv.  =  0.552  mg.  for  Pd  +  +  ions. 
Elec.  Potential  =  <  -0.793  volt.  Overvoltage  of  H2  =  0.24- 
0.46  volt. 

The  experiments  carried  out  in  the  author's  laboratory  by  the 
older  methods  were  not  successful  for  the  quantitative  estimation 
of  this  metal.  It  was  only  when  R.  Amberg,*  at  the  author's 
suggestion,  experimented  with  a  rapidly  rotating  anode  that  it  was 
found  possible  to  obtain  a  firmly  adherent  deposit  of  palladium. 

If  the  palladium  salt  is  soluble  in  water,  enough  sulphuric  acid 
is  added  to  the  solution  so  that  120  cc.  of  electrolyte  will  contain 
about  30  per  cent  of  concentrated  acid  and  the  electrolysis  is 
carried  out  in  a  solution,  which  is  not  hotter  than  65°,  with  an 
initial  electromotive  force  of  0.75  volt;  toward  the  end  of  the 
operation,  a  current  of  1.15  volts  is  used  but  if  the  potential  is 
increased  above  this  value  a  spongy  deposit  will  be  obtained. 
About  0.3  gm.  of  palladium  will  be  deposited  quantitatively  in 
4  to  6  hours.  As  cathode  the  roughened  platinum  dish  is  used 
and  as  anode  a  platinum  disk,  made  to  revolve  from  600  to  1000 
times  a  minute. 

To  test  the  solution  for  palladium  at  the  end  of  the  electrolysis, 
a  little  of  the  electrolyte  is  treated  with  potassium  iodide;  a 


*  Z.  Elektrochem.,  10,  385,  853  (1904). 


PALLADIUM  143 

brown  precipitate  or  coloration  of  palladous  iodide,  Pdl,  will  be 
formed  by  palladium.  The  coloration  does  not  disappear  upon 
the  addition  of  sulphurous  acid;  if  this  is  the  case,  the  color  was 
due  to  free  iodine. 

If  the  test  shows  no  palladium,  the  current  is  turned  off,  the 
liquid  is  poured  out  of  the  dish  and  the  deposit,  after  the-  usual 
washing  with  water  and  alcohol,  is  dried  at  110°. 

For  a  satisfactory  deposition  of  this  metal  it  is  important  that 
the  potential  of  the  current  does  not  rise  above  1.15  volts.  If 
the  electrolysis  is  conducted  with  a  current  of  NDi00  =  0.05  to 
0.04  ampere,  the  potential  rises,  after  the  greater  part  of  the  metal 
has  deposited,  to  more  than  1.15  volts.  It  is  then  necessary, 
by  changing  a  front  switch  and  a  shunt  resistance,  to  diminish 
the  current  strength  enough  so  that  the  final  potential  is  not  over 
1.15  volts.  In  this  way  the  current  is  reduced  to  0.01  or  0.02  am- 
pere. '  If  the  voltage  is  allowed  to  exceed  1.15  volts,  hydrogen  is 
evolved  at  the  cathode  and  a  spongy  deposit  of  palladium  is 
formed  (p.  88).  When  the  current  has  been  reduced  to  0.01  or 
0.02  ampere,  the  deposition  of  the  metal  is  practically  complete 
and  the  current  is  allowed  to  continue  only  until  the  electrolyte 
has  cooled  to  about  the  temperature  of  the  air. 

Palladium  salts  which  are  insoluble  in  water  are  dissolved  in  as 
little  concentrated  sulphuric  acid  as  possible  and  diluted  with  water 
and  enough  more  sulphuric  acid  is  added  to  give  the  proper  acidity. 
The  acid  content  of  about  30  per  cent  sulphuric  acid  is  the  most 
suitable  because  this  acid  has  the  greatest  conductivity  as  Grotrian  * 
has  pointed  out.  The  presence  of  the  palladium  salt  has  little 
effect  upon  the  conductivity  of  such  a  solution. 

To  remove  the  palladium  deposit  from  the  platinum  dish,  it 
is  treated  with  a  solution  of  potassium  chloride,  saturated  at  the 
room  temperature,  and,  after  heating  to  70°  or  80°,  a  little  solid 
chromic-acid  anhydride  is  added  while  the  dish  is  kept  in  constant 
motion  so  that  the  air  comes  in  contact  with  the  metal.  In  this  way 
the  solution  of  the  palladium  is  effected  without  dissolving  much 
platinum. 

*  Poggendorff's  Ann.,  161,  378  (1874). 


144          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Rhodium. 
At.  Wt.  =  102.9    Elec.  Equiv.  =  0.356  mg.  for  Rh+++  ions. 

Rapid  Precipitation  of  Rhodium  from  Sulphuric-acid  Solution. 

Julia  Langness  *  used  as  electrolyte  a  solution  of  sodium-rhodium 
chloride,  Na2RhCle,  which  contained  about  0.058  gm.  of  rhodium 
and  2.5  cc.  of  sulphuric  acid  (1  :  10).  The  solution  was  diluted 
with  water  to  a  volume  of  about  105  cc.  and  electrolyzed  in  a 
silver-coated  platinum  dish  with  a  spiral  anode  making  600  revolu- 
tions per  minute  and  a  current  of  8  or  9  amperes  and  7  to  8  volts. 
The  deposition  of  this  amount  of  metal  required  7  to  10  minutes. 

By  using  a  sieve  anode  (p.  56)  it  was  found  possible  to  deposit 
twice  as  much  metal  in  the  same  time  with  a  current  of  7  volts 
and  15  amperes.  In  this  case  the  volume  of  the  solution  was 
60  cc. 

*  J.  Am.  Chem.  Soc.,  29,  469  (1902). 


BISMUTH  145 


Bismuth. 

At.    Wt.  =  208.0     Elec.    Equiv.  =  0.718   mg.  for  Bi+++  ions. 
Elec.  Potential  =  <  -  0.393  volt. 

Most  methods  proposed  for  the  electrolytic  determination 
of  bismuth  are  more  or  less  unreliable,  partly  owing  to  the 
difficulty  in  obtaining  satisfactory,  adherent  deposits  and  partly 
owing  to  the  tendency  of  some  bismuth  peroxide  to  deposit  on 
the  anode.  Of  the  methods  that  have  given  satisfactory  results 
with  stationary  electrolytes,  the  following  one  devised  by  O. 
Brunck  *  is  to  be  recommended.  If,  however,  a  quick  method 
as  well  as  an  accurate  one  is  desired,  it  is  better  to  stir  the  elec- 
trolyte. 

The  conditions  under  which  Brunck  obtained  good  results 
comprised  the  use  of  a  platinum  gauze  electrode  (p.  59),  a  maxi- 
mum potential  of  2  volts,  a  moderate  amount  of  acid,  and  heating 
the  solution  before  starting  the  electrolysis.  A  nitric-acid  solu- 
tion was  used,f  containing  enough  free  acid  to  prevent  the  precipi- 
tation of  basic  salt  upon  dilution  to  a  volume  of  about  100  cc. 
The  quantity  of  acid  must  not  exceed  2  per  cent  or  the  metal  will 
be  deposited  in  a  crystalline  condition  such  that  there  are  losses 
during  the  washing.  A  larger  quantity  of  acid  may  also  cause 
the  formation  of  bismuth  peroxide.  As  a  source  of  current 
which  must  not  exceed  a  potential  of  2  volts  at  any  time,  a  single 
accumulator  cell  may  be  used,  or  several  cells  connected  in  parallel. 
The  solution  is  heated  nearly  to  boiling  before  turning  on  the 
current  but  the  flame  is  removed  after  the  electrolysis  is  started. 
If  more  than  0.1  gm.  of  bismuth  is  present  in  100  cc.  of  the  solution, 
the  current  density  may  be  NDioo  =  0.5  ampere,  or  even  more, 
but  if  less  than  0.5  gm.  bismuth  is  present  it  is  better  not  to  use 
over  0.1  ampere.  Here  the  current  density  at  the  start  is  under- 
stood. As  the  solution  cools  and  as  it  becomes  impoverished  of 
metal  ions,  the  current  density  naturally  falls  and  at  the  last 
amounts  to  not  more  than  a  few  hundredths  of  an  ampere.  It 
is  not  possible  to  have  it  otherwise  if  the  work  is  to  be  carried  out 
at  a  constant  voltage.  The  electrodeposition  of  0.3  gm.  of 

*  Ber.,  35,  1871  (1902). 

f  For  practice  either  pure  bismuth  may  be  dissolved  in  nitric  acid  or 
basic  bismuth  nitrate  may  be  dissolved  in  dilute  nitric  acid. 


146  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

bismuth,  however,  does  not  require  more  than  3  hours.  The 
presence  of  a  little  sulphuric  acid  does  not  have  any  appreciable 
effect  upon  the  analysis. 

The  favorable  results  obtained  with  other  metals,  during  the 
last  few  years,  by  keeping  the  electrolytes  in  constant  motion  led 
to  the  expectation  that  the  deposition  of  bismuth  could  also  be 
made  more  favorable  by  the  use  of  the  new  method.*  The  results 
obtained  were  not  satisfactory  at  first.  Thus  the  method  proposed 
by  K.  Wimmenauerf  did  not  prove  wholly  successful  in  the  hands 
of  other  experimenters. {  It  was  only  after  Haber,§  Le  Blanc  and 
others  had  called  attention  to  a  new  point  of  view  which  had 
hitherto  been  unnoticed  in  electro-analysis  that  it  was  found 
possible  to  work  out  the  rapid  electrodeposition  of  bismuth  upon 
a  scientific  basis.  The  new  feature  consists  in  the  measurement 
and  control  of  the  cathode  potential  under  which  the  bismuth 
is  deposited.  The  consideration  of  this  electrical  factor  marks 
a  new  era  in  electro-analytical  investigation. 

Since  the  abandonment  of  purely  empirical  methods  by  which 
analyses  were  made  with  a  certain  number  of  galvanic  elements, 
the  path  taken  by  investigation  in  this  field  has  been  characterized 
by  a  number  of  important  innovations.  Classen  introduced  the 
use  of  accumulators  and  measuring  instruments  and  emphasized 
the  importance  of  the  current  density  in  electrolysis.  Kiliani, 
and  after  *him  Freudenberg,  attempted  to  effect  separations  of 
metals  by  maintaining  a  certain  difference  in  potential  between  the 
electrodes,  a  process  which,  in  accord  with  the  discovery,  by  Nernst 
and  Caspari,  of  the  overvoltage  of  hydrogen,  must  be  modified 
and  has  not  proved  to  be  universally  applicable.  If,  in  such  an 
analysis,  only  the  potential  difference  between  the  electrodes  is 
measured,  the  fact  is  not  taken  into  consideration  that  this  total 
difference  in  voltage  is  the  sum  of  the  drops  in  potential  at 
the  cathode  and  at  the  anode  and  that  these  two  quantities  are 

*  General  references  to  the  literature  covering  the  rapid  electrodeposition 
and  separation  of  bismuth  in  various  solutions:  Exner,  J.  Am.  Chem.  Soc., 
26,  896  (1903);  A.  Fischer  and  Boddaert,  Z.  Elektrochem.,  10,  945  (1904); 
H.  J.  S.  Sand,  J.  Chem.  Soc.,  London,  91,  373  (1907);  A.  Fischer,  Chem.  Ztg., 
31,  25  (1907);  Z.  Elektrochem.,  13,  469  (1907);  Smith  and  Kollock,  J.  Am. 
Chem.  Soc.,  27,  1527  (1905). 

t  Z.  anorgan.  Chem.,  27,  1  (1901). 

t  Cf.  A.  Fischer  and  R.  J.  Boddaert,  Z.  Elektrochem.,  10,  945  (1904). 

§  Z.  phys.  Chem.,  32,  194  (1900). 


BISMUTH  147 

independent  of  one  another.  When  the  metal  is  deposited  upon 
the  cathode  it  is  obvious  that  particular  stress  should  be  laid  upon 
the  drop  in  potential  at  this  electrode,  for  it  is  possible  that  the 
reactions  taking  place  at  the  two  electrodes  may  change  consider- 
ably during  the  electrolysis  without  there  being  any  perceptible 
change  in  the  potential  difference  between  the  two  electrodes.  In 
this  way  the  cathode  potential  may  become  quite  different  from 
the  value  necessary  for  the  satisfactory  deposition  of  a  metal. 
Many  separations  are,  indeed,  successful  without  taking  these  sin- 
gle potentials  into  consideration,  but  this  is  due  either  to  the  fact 
that  large  differences  in  the  cathode  potential  have  little  effect 
upon  the  satisfactory  deposition  of  the  metal  in  question  or  that, 
owing  to  the  addition  of  certain  substances  which  have  been  found 
by  experiment  to  be  helpful,  the  nature  of  the  electrolyte  is  such 
that  the  cathode  potential  is  kept  within  the  necessary  limits 
throughout  the  process. 

H.  J.  S.  Sand  has  found  that  the  control  of  the  cathode  potential 
is  especially  important  for  the  successful  analysis  of  a  bismuth 
solution  by  electrolysis.  At  first  sight  the  necessity  of  making 
such  measurements  during  the  progress  of  an  analysis  seems  to 
introduce  an  inconvenient  and  time-consuming  complication. 
When  one  considers,  however,  that,  owing  to  the  advantage  gained 
by  thoroughly  stirring  the  electrolyte,  the  precipitation  of  0.32 
to  0.38  gm.  of  bismuth  need  not  require  more  than  10  or  15 
minutes,  then  the  above  objection  is  removed.  Moreover,  when 
the  apparatus  is  once  set  up  for  measuring  and  regulating  the 
cathode  potential,  it  does  not  make  any  serious  demands  on  the 
chemist. 

In  accordance  with  what  has  already  been  said,  Sand's  method 
for  depositing  bismuth  can  be  easily  explained.  \\|hen  the  elec- 
tric current  is  passed  through  an  acid  solution  containing  bis- 
muth, then,  as  soon  as  the  voltmeter  registers  a  voltage  higher 
than  the  discharge  potential  of  bismuth  ions,  metallic  bismuth 
will  begin  to  separate  upon  the  cathode  and  in  a  satisfactory 
condition.  At  the  same  time  the  ammeter  registers  a  certain 
current  strength  and  the  single  potential  of  the  bismuth  at  the 
cathode  must  have  a  certain  value;  but  what  this  exact  value  is 
we  do  not  need  to  know.  After  a  short  time  has  elapsed,  tlie 
solution  becomes  poorer  in  bismuth  ions  and  then  the  cathode 
potential  must  rise  in  accordance  with  Nernst-s  formula  (cf.  p.  89), 


148 


QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 


Since  the  cathode  potential  of  bismuth  is  not  much  below  that  of 
hydrogen,  which  is  taken  as  zero,  this  increase  in  potential  will 
cause  evolution  of  hydrogen  and,  as  bismuth  is  influenced  more 
than  almost  any  other  metal  by  this  simultaneous  discharge  of 
hydrogen,  the  deposit  will  form  in  a  spongy  condition.  To 
prevent  this,  it  is  necessary  to  keep  the  cathode  potential  lower 
than  the  discharge  potential  of  hydrogen  ions.  The  usual  measur- 
ing instruments  do  not  give  enough  information  to  accomplish 
this;  the  voltmeter  only  registers  the  total  difference  in  potential 
between  the  electrodes  and  this  may  be  influenced  by  the  reac- 
tions taking  place  at  the  anode.  It  is  necessary,  therefore,  to 
provide  some  means  of  measuring  the  drop  in  potential  at  the 
cathode  and  if  we  know  the  values  which  are  favorable  for  a  good 
deposition  of  metal,  we  are  then  able  to  keep  the  potential  within 
the  permissible  limits. 

A  diagram  of  Sand's  apparatus  is  shown  in  Fig.  47.  S  repre- 
sents a  beaker  containing  the  bismuth  solution  and  the  Cylinder- 
shaped  platinum  gauze  electrodes  a,  c  (see  also  Figs.  31  and  32). 
The  electrodes  are  'connected  in  the  usual  manner  with  the  source 
of  the  current,  Q  (accumulators,  p.  68)  to  be  used  in  the  analysis. 
The  capillary  tube  of  the  auxiliary  electrode  K  dips  into  the  solu- 
tion and  the  opening  e  is  brought  close  to  the  cathode  c.  In  this 
way  a  compound  element  is  obtained,  consisting  on  one  side  of 


the  electrolyte  of  S  and  its  pole  c;  the  latter,  as  soon  as  the  elec- 
trolysis is  in  progress,  is  represented  by  the  bismuth  deposit  upon 
the  cathode.  On  the  other  side,  the  mercury  at  the  bottom  of  K 
forms  the  second  pole  and  is  in  contact  with  the  liquid  in  the 
apparatus  K  which  is  also  contained  in  the  capillary  tubing.  Thus 


BISMUTH  149 

the  two  liquids  are  in  contact  at  the  opening  e  of  the  capillary 
(cf.  p.  41). 

If,  now,  we  connect  the  mercury  in  K  with  the  end  B  of  the 
slide-wire  bridge  BC,  which  is  rolled  upon  a  cylinder,  and  on  the 
other  side  connect  the  gauze  electrode  c,  through  the  capillary 
electrometer  E,  with  the  sliding  contact  D,  then  we  can  com- 
pensate the  electromotive  force  Kec  by  the  opposite  electromotive 
force  that  corresponds  to  the  potential  difference  between  B  and 
D.  To  accomplish  this  it  is  merely  necessary  to  move  the  sliding 
contact  to  a  point  D  on  the  wire  BC  which  causes  the  surface  of 
the  mercury  in  the  capillary  electrometer  to  rest  at  the  zero  point. 
The  value  of  this  electromotive  force  will  be  read  directly  in  volts 
by  inserting  a  voltmeter  V  between  the  points  B  and  D.  The 
way  this  compensating  current  is  fed  by  a  special  accumulator 
is  described  on  page  42. 

The  reading  at  the  voltmeter  does  not  give  the  single  poten- 
tial of  the  bismuth  at  the  cathode  c  but  it  shows  the  difference 
in  potential  between  the  cathode  c  and  the  mercury  in  K.  To 
compute  the  potential  at  c  it  would  be  necessary  to  know  the 
potential  of  the  mercury  electrode  K.  It  is  not  at  all  necessary 
to  know  this,  however,  for  it  suffices  to  know  that  the  potential 
in  the  auxiliary  electrode  remains  constant,  for  then  any  change 
in  the  reading  of  the  voltmeter  V  will  show  that  a  change  has 
taken  place  in  the  potential  at  c. 

It  is  necessary,  then,  to  determine  experimentally  what  the 
reading  of  the  voltmeter  should  be  to  cause  the  bismuth  to  deposit 
in  a  satisfactory  condition.  If  this  potential  is  once  known,  it  is 
only  a  question  of  carrying  out  the  electrolysis  so  that  the  poten- 
tial at  the  cathode  remains  constant,  or  rather  that  it  does  not 
vary  except  within  the  allowable  limits.  To  keep  the  potential 
absolutely  constant  is  out  of  the  question  here,  as  in  all  other 
methods. 

The  above  description,  based  upon  the  sketch  shown  in  Fig.  47, 
will  enable  one  to  understand  the  picture  of  the  apparatus  shown 
in  Plate  II.  Details  of  the  connections  will  be  evident  from  a 
study  of  Fig.  48,  where  the  lettering  corresponds  to  that  used  in 
Plate  II  (back  of  the  book). 

The  three  electric  circuits  shown  in  Fig.  48,  which  are  the  same 
as  those  referred  to  in  Fig.  47  although  the  lettering  is  different, 
are  as  follows: 


150 


QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 


1.  The  wires  from  the  main  circuit  to  the  accumulator  A  and 
the  slide-wire  bridge  C  (in  which  the  commutator  switch  B  is  also 
included)  are  shown  by  the  heavy  black  lines.     This  circuit  is 
AaCbBcA. 

2.  The  circuit  which  branches  off  from  the  lead  wires  and  con- 
tains the  voltmeter  E  (also  a  resistance  D)  is  shown  by .  heavy 
dashes  and  is  CdDeEfC. 

3.  The  circuit  which  contains  the  auxiliary  electrode  L  and 
the  electrolytic  cell  K  is  shown  by  dotted  lines  CgLKhikGlmHnC. 

In  the  last  circuit  is  found  the  capillary  electrometer  G.     This 


FIG.  48. 

instrument  must  be  short  circuited  when  not  in  use.  There  is, 
therefore,  a  key  H  connected  with  the  last  of  the  above-mentioned 
circuits,  which  is  arranged  so  that  when  the  capillary  electrometer 
is  not  being  used  the  short  circuit  GlmHokG  is  formed.  If,  how- 
ever, it  is  desired  to  make  a  measurement,  the  finger  is  pressed 
against  the  key  of  H  and  then  the  connection  o,  represented  by 
heavy  dots  and  dashes  in  Fig.  48,  is  broken  and  the  circuit  becomes 
that  given  above. 

The  bismuth  determination  is  carried  out  as  follows :  *  The 
solution,  containing  0.2  to  0.3  gm.  of  bismuth  and  about  2.5  cc. 
of  nitric  acid  (sp.  gr.  1.4),  is  in  the  beaker  S  (Fig.  47).  It  is 
heajbed  and  a  solution  of  8  gms.  sodium  tartrate  in  water  is  added 

*  The  description  given  here  is  based  upon  that  published  by  Sand. 
A.  Fischer  has  tested  the  method  in  the  author's  laboratory  under  the  con- 
ditions described  by  Sand  and  has  obtained  good  results.  The  apparatus  used 
in  these  experiments  differs  from  that  of  Sand  inasmuch  as  the  two  electrodes 
were  stationary  and  the  electrolyte  was  kept  in  motion  by  an  independent 
stirrer  (cf.  p.  66).  The  capillary  electrometer  may  be  either  to  the  right  or 
to  the  left  of  K. 


BISMUTH  151 

with  enough  more  water  to  make  the  total  volume  100  cc.  The 
anode  a  and  the  weighed  cathode  c  are  placed  in  the  solution 
and  connected  with  the  binding  posts  but  the  current  is  not  yet 
turned  on.  By  means  of  a  small  flame  beneath  the  beaker,  the 
temperature  of  the  liquid  is  kept  practically  constant  through  the 
entire  operation  but  it  is  not  necessary  to  use  a  thermometer. 

After  the  stopcock  of  the  auxiliary  electrode  has  been  turned 
so  that  a  few  drops  of  sodiurn-sulphate  solution  run  out,  in  order 
to  be  certain  that  this  solution  fills  the  entire  capillary  tube,  the 
cock  is  turned  back  and  the  capillary  tubing  is  sunk  into 
the  solution  to  be  analyzed  so  that  its  end  e  lies  very  close  to  the 
cathode  (Fig.  47).  The  wire  stem  of  the  cathode  (Fig.  32)  is 
connected  with  the  capillary  electrometer  and  the  remaining  con- 
nections are  made  in  accordance  with  Fig.  48  and  Plate  II. 

When  everything  is  ready,  the  stirrer  is  set  hi  motion  (900  to 
1000  revolutions  per  minute)  and  the  current  for  the  analysis 
is  turned  on  (Fig.  47;  see  also  Fig.  39).  The  bismuth  at  once 
begins  to  deposit  upon  the  cathode.  The  sliding  contact  D  is 
now  moved,  by  the  aid  of  the  knob  on  the  cylinder  (Plate  II), 
until  the  voltmeter  registers  about  0.63  volt,  and  the  position  of 
the  mercury  thread  in  the  capillary  electrometer  is  watched  to 
see  whether  the  surface  of  the  mercury  is  at  rest  at  b  (Fig.  47) 
when  the  key  H  (Plate  II)  is  pressed.  By  slightly  changing  the 
sliding  contact  D  it  is  easy  to  bring  the  mercury  to  the  zero  posi- 
tion. 

Under  these  conditions  the  deposition  of  the  bismuth  takes 
place  in  the  most  favorable  manner  and  it  now  is  only  a  matter 
of  keeping  the  cathodic  potential  as  nearly  constant  as  possible. 
According  to  the  explanation  on  page  41,  it  is  evident  that  the 
potential  will  tend  to  rise  and  this  must  be  offset  by  lessening 
the  strength  of  the  current,  for  the  principle  of  the  method  lies 
in  carrying  out  the  analysis  at  a  practically  constant  potential. 
During  the  ten  or  fifteen  minutes  required  for  the  analysis,  it  is 
necessary  to  keep  diminishing  the  current  until  toward  the  end 
only  about  0.2  ampere  is  used,  but  it  is  permissible  to  allow  the 
potential  to  rise  as  high  as  0.9  volt. 

When  the  potential  and  the  current  strength  have  reached  their 
constant  final  values,  i.e.,  when  the  former  stops  rising  so  that  it 
is  no  longer  necessary  to  diminish  the  current,  the  greater  part 
of  the  metal  will  have  been  deposited.  If  now  the  analysis  is 


152  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

continued  for  about  half  as  long  again  as  the  time  from  the  start 
of  the  electrolysis,  one  may  be  certain,  from  theoretical  reasons, 
that  all  the  metal  will  be  precipitated.  To  make  sure  of  this, 
however,  a  little  of  the  liquid  is  removed  from  the  beaker  and 
tested  with  ammonium  sulphide. 

A  good  way  to  conduct  the  washing  of  the  deposited  metal  is 
first  to  remove  the  auxiliary  electrode,  without  interrupting  the 
current,  then  to  stop  the  stirrer  and  quickly  remove  the  beaker 
containing  the  electrolyte  from  under  the  electrodes,  replacing  it 
with  a  beaker  of  distilled  water.  By  starting  the  stirrer  again, 
a  thorough  washing  of  the  deposit  is  obtained  in  a  few  seconds,  for 
in  this  way  the  water  is  forced  very  energetically  through  the 
meshes  of  the  gauze  electrode.  The  stirrer  is  now  stopped  and 
the  current  turned  off;  it  is  then  only  necessary  to  rinse  off  the 
cathode  once  with  distilled  water,  dip  it  in  alcohol,  dry  and  weigh. 

After  the  capillary  tube  of  the  auxiliary  electrode  has  been 
rinsed  with  water,  a  few  drops  of  sodium-sulphate  solution  are 
allowed  to  flow  into  it,  as  described  on  page  41,  to  remove  any 
liquid  that  may  have  diffused  into  the  tubing  from  the  beaker. 

In  the  above  directions,  it  was  recommended  to  allow  the 
current  to  pass  through  the  solution  for  half  as  long  again  as  the 
tune  between  the  start  of  the  electrolysis  and  the  point  where 
the  voltage  and  amperage  had  reached  their  final  values.  Ac- 
cording to  the  explanation  on  page  89,  the  concentration  of  the 
electrolyte  will  have  been  diminished  to  an  immeasurably  small 
value  as  soon  as  the  potential  has  been  raised  about  0.2  volt. 
It  is  well,  however,  to  let  the  current  act  a  little  longer. 

Another  method  for  determining  bismuth  electrolytically,  and 
one  suitable  for  small  quantities  of  this  element,  will  be  given 
under  the  section  on  the  analysis  of  commercial  copper. 


ANTIMONY  153 

Antimony. 

At.  Wt.  =  120.2.  Elec.  Equiv.  =  0.415  mg.  for  Sb+++  ions. 
Elec.  Potential  =  <  -  0.463  volt. 

The  only  reliable  method  for  the  electrolytic  determination  of 
antimony  is  from  a  solution  of  the  thio  salt.*  At  the  same  time 
a  separation  of  antimony  from  tin  and  arsenic  may  be  made  by 
carrying  out  the  special  conditions  described  in  the  section  of 
this  book  dealing  with  separations.  The  objection  to  the  method, 
however,  is  the  fact  that  if  the  electrolysis  is  continued  too  long, 
e.g.,  overnight,  polysulphides  are  formed  from  the  sodium  sulphide 
in  the  solution  and  these  polysulphides  exert  a  solvent  effect  upon 
the  metallic  antimony.  The  chemical  and  electrolytic  behavior 
of  the  antimony  in  this  determination  will  be  explained  and  ways 
and  means  will  be  shown  for  meeting  the  above  objection. 

Antimony  pentasulphide  dissolves  in  sodium  sulphide  as  repre- 
sented by  the  equation: 


and  the  sodium  thioantimonate  is  dissociated  thus  : 

Na3SbS4  <=±  3  Na+  +  SbS™~ 

The  antimony  is  present,  therefore,  as  a  component  of  the 
complex  SbSI  anion  and  it  is  to  be  expected,  therefore,  that 
the  antimony  in  this  negatively  charged  complex  will  at  first 
migrate  to  the  anode,  when  the  current  is  turned  on.  To  study 
the  migration  relations  of  antimony  during  the  electrolysis,  H. 
Ost  and  W.  Klapproth  f  separated  the  region  of  the  anode  in  the 
cell  from  the  cathode  region  by  interposing  a  diaphragm  of  porous 
clay  and  with  such  an  apparatus  the  following  experiments  were 
performed. 

1.  First,  the  electrolyte  was  distributed  uniformly  in  the  anode 
and  cathode  compartments  and  subjected  to  electrolysis;  as  a 
result  all  the  metal  in  the  cathode  compartment  was  deposited 
upon  the  cathode  while  the  anode  compartment  contained  prac- 
tically all  the  antimony  that  was  originally  present  there.  Thus 
no  antimony  ions  migrated  from  the  anode  compartment  into  the 
cathode  compartment. 

*  Methods  of  A.  Classen:   Classen  and  v.  Reis,  Ber.,  14,  1622  (1881);   17, 
2467  (1884);  18,  1104  (1885);  Classen,  ibid.,  27,  2060  (1894). 

*  Z.  angew.  Chem.,  1900,  827. 


154  QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 

2.  The  entire  antimony  solution  was  placed  in  the  cathode 
compartment  and  the  anode  compartment  was  filled  with  pure 
sodium  -sulphide  *  solution.     The  result  of  the  electrolysis  was  a 
quantitative  deposition  of  the  antimony  upon  the  cathode  and 
ht)  antimony  solution  reached  the  anode  compartment. 

3.  As  a  final  experiment,  the  entire  antimony  solution  was 
placed  in  the  anode  compartment  and  the  cathode  compartment 
was  filled  with  sodium-sulphide  solution;   under  these  conditions 
there  was  no  trace  of  antimony  deposition  but  antimony  sulphide 
was  deposited  upon  the  anode. 

The  influence  of  the  sodium-sulphide  solution  was  also  studied. 
If  at  the  start  the  antimony  is  all  present  in  the  cathode  compart- 
ment together  with  an  excess  of  sodium  sulphide,  then,  as  in  the 
second  experiment,  all  the  antimony  is  deposited  upon  the  cathode; 
the  potential  of  the  bath  is  low  on  account  of  the  high  concentra- 
tion of  sodium  sulphide.  If  the  solution,  however,  contains  but 
little  sodium  sulphide,  then  the  potential  of  the  bath  Becomes 
high  and  a  little  of  the  antimony  passes  through  the  diaphragm 
and  deposits  upon  the  anode  as  antimony  sulphide. 

From  these  experiments  it  is  apparent  that  under  the  usual 
experimental  conditions,  i.e.,  in  the  presence  of  considerable 
sodium  sulphide,  the  antimony  neither  migrates  from  the  anode 
into  the  cathode  space  nor  does  it  migrate  in  the  opposite  direction. 
In  other  words,  it  does  not  take  part  at  all  in  the  conductance 
of  the  current  through  the  solution.  The  action  of  the  current, 
according  to  Ost  and  Klapproth,  consists  essentially  of  the  decom- 
position of  sodium  sulphide: 

Na2S  =  2  Na  +  S, 

and  the  deposition  of  antimony  upon  the  cathode  is  really  the 
result  of  a  secondary  reaction,  which  is  the  action  of  the  discharged 
sodium  ions  upon  the  sodium  thioantimonate : 

Na3SbS4  +  5  Na  =  Sb  +  4  Na^S. 

As  regards  the  reactions  at  the  anode,  it  has  been  found  that  in 
the  first  stages  of  the  electrolysis  only  sulphur  ions  from  the 
sodium  sulphide  are  discharged  there  and  the  sulphur,  as  fast 
as  it  is  set  free,  combines  with  the  sodium  sulphide  to  form  sodium 
polysulphide : 

+  S  = 


ANTIMONY  155 

Later  on,  oxygen  is  liberated  at  the  anode  which  also  acts  upon 
sodium  hydrogen  sulphide  *  to  form  polysulphide  :  f 
6NaSH  +  3O  =  3  Na^S,  +  3  H20. 

This  polysulphide  gradually  diffuses  during  the  progress  of  the 
analysis,  and  if  the  space  around  the  cathode  is  not  separated  from 
the  rest  of  the  solution  it  begins  to  dissolve  the  deposited  anti- 
mony as  soon  as  it  reaches  the  cathode  : 

2Sb  +  3Na2S2  =  2Na3SbS3. 

This  was  a  frequent  cause  of  failure  in  antimony  determinations 
when  the  current  was  passed  through  the  cell  for  too  long  a  time, 
and,  to  prevent  the  diffusion  of  the  polysulphide,  Ost  and  Klap- 
proth  recommended  that  the  cell  be  separated  into  two  compart- 
ments by  a  diaphragm.  In  this  way  good  results  were  obtained. 

This  complication  of  the  apparatus  is  unnecessary,  however,  if 
some  substance  is  added  to  the  bath  which  acts  upon  the  poly- 
sulphide and  reduces  it  to  monosulphide. 

LecrenierJ  used  sodium  sulphite  for  this  purpose.     It  reacts 
with  polysulphides  to  form  thiosulphate  and  monosulphide: 
Na2S2  +  Na,2SO3  =Na2S2O3  +  Na2S. 

Quite  independent  of  one  another,  Hollard  and  Bertiaux,  as  well 
as  A.  Fischer,  have  used  potassium  cyanide  for  the  same  purpose 
since  the  year  1900.  By  this  salt,  the  polysulphide  is  reduced  to 
monosulphide  and  the  cyanide  is  changed  to  thiocyanate: 

Na2S2  +  KCN  =  KCNS  +  Na-jS. 

Sodium  hydrosulphite  has  also  been  tried  by  A.  Fischer  and 
found  to  work  satisfactorily  but  neither  this  reagent  nor  the  above- 
mentioned  sodium  sulphite  has  any  advantages  over  potassium 
cyanide.  § 

In  recent  years  the  electrolytic  determination  of  antimony  from 
sodium-sulphide  solutions  has  been  examined  critically  by  a  num- 

*  NaSH  is  formed  by  the  hydrolysis  of  some  of  the  sodium  sulphide: 
Na2S  +  H2O  =  NaHS  +  NaOH. 

t  Some  thiosulphate  is  also  formed  but  it  does  no  harm: 

+  30  =  N*8A. 


t  Chem.-Ztg.,  13,  1219  (1889). 

§  The  potassium  cyanide  has  the  advantage  of  converting  traces  of  copper 
into  complex  ions  which  are  not  decomposed  during  the  analysis. 


156  QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 

ber  of  investigators  *  and  the  general  opinion,  obtained  as  a  result 
of  such  studies,  is  that  the  method  usually  gives  results  that  are 
a  little  higher  than  the  truth  and  that  the  positive  error  is  appar- 
ently caused  by  small  quantities  of  oxygen  and  of  sulphur  present 
in  the  antimony  deposit,  f  It  has  been  found  that  when  the 
electrolysis  is  carried  out  in  hot  solutions  and  with  high  current 
density  the  results  are  a  little  higher  than  when  the  analysis  is 
made  at  room  temperature  with  a  weaker  current.  According 
to  several  authorities,  the  positive  error  amounts  to  from  1  to  1.5 
per  cent  J  and  for  this  reason  Henz  proposed  that  a  deduction  of 
1.5  per  cent  be  made  upon  the  weight  of  deposit  actually  obtained. 

At  the  author's  suggestion,  Dr.  Scheen  §  has  carried  out  some 
experiments  to  ascertain  the  cause  of  these  high  results  and  he 
has  found  that  the  error  is  really  due  to  inclusions  which  are 
dependent  upon  the  nature  of  the  cathode  surface.  It  was 
mentioned  on  page  58  that  a  rough  platinum  surface  was  more 
likely  to  give  rise  to  this  sort  of  an  error  and  it  has  been*  found 
that  gauze  electrodes  are  particularly  bad  in  this  respect.  The 
facts  mentioned  by  other  authors  were  confirmed,  that  the  temper- 
ature of  the  electrolyte  should  not  exceed  65°  to  70°  and  that 
the  presence  of  considerable  alkali  hydroxide  tends  to  increase  the 
positive  error.  The  temperature  plays  a  part  in  this  behavior, 
because  above  70°  the  deposit  is  rather  spongy  and  has  a  greater 
tendency  to  take  up  foreign  substances.  If  the  electrolyte  con- 
tains considerable  alkali  hydroxide,  the  deposit  will  retain  an 
alkaline  odor  even  after  the  most  careful  washing  with  water  and 
alcohol. 

The  author  originally  recommended  polished  platinum  dishes 
for  this  determination  although  they  will  not  hold  firmly  much 
more  than  from  0.1  to  0.15  gm.  of  antimony  deposit.  For  this 
reason  he  was  led  to  adopt  dishes  with  the  inside  surface  rough- 
ened. The  experiments  performed  by  Dr.  Scheen  with  both  smooth 
and  roughened  dishes  have  shown,  however,  that  correct  results 

*  F.  Henz,  Z.  anorg.  Chem.,  37,  1  (1903);  F.  Foerster  and  J.  Wolf,  Z.  Elek- 
trochem.,  13,  205  (1907);  H.  J.  S.  Sand,  ibid.,  326;  J.  M.  M.  Dormaar,  Z. 
anorg.  Chem.,  63,  349  (1907). 

t  Foerster  was  inclined  to  believe  that  a  solid  solution  of  antimony  oxide 
and  of  antimony  sulphide  in  metallic  antimony  was  formed. 

t  If  the  electrolyte  contains  more  than  3  per  cent  of  alkali  hydroxide,  the 
positive  error  may  be  3  per  cent  of  the  weight  of  the  deposit. 

§  Z.  Elektrochem.,  14,  257  (1908). 


ANTIMONY  157 

can  be  obtained  with  the  former,  whereas,  with  the  latter,  or  with 
gauze  electrodes  of  various  designs,  the  results  are  too  high. 
When  most  accurate  results  are  desired,  therefore,  roughened 
dishes  or  gauze  electrodes  should  not  be  used.  Treatment  of 
the  dishes  with  dilute  aqua  regia  serves  to  etch  them  very  slightly 
and  such  dishes  will  hold  a  deposit  of  as  much  as  0.3  gin.,  although 
it  is  better  not  to  have  more  than  0.2  gm.  of  antimony  in  the  solu- 
tion. 

Procedure  for  Depositing  Antimony  from  a  Sodium-sulphide 

Solution. 

For  this  method  *  it  is  immaterial  whether  the  dissolved  anti- 
mony is  present  in  the  trivalent  or  quinquevalent  condition.  In 
the  course  of  a  chemical  analysis,  the  antimony  is  usually  obtained 
as  the  trisulphide  or  pentasulphide,  either  by  direct  precipitation 
or  as  a  result  of  a  separation  from  other  sulphides. f 

The  antimony  sulphide  is  dissolved  in  about  80  cc.  of  a  solu- 
tion which  has  been  saturated  with  crystallized  sodium  sulphide 
at  the  room  temperature  (the  specific  gravity  of  such  a  solution 
is  1.14),  30  cc.  of  a  freshly  prepared  potassium-cyanide  solution 
are  added,  and  the  mixture  is  diluted  with  water  to  a  total  volume 
of  120  to  140  cc. 

The  solution  is  electrolyzed  at  a  temperature  of  65°  (not  over 
70°  in  any  case)  with  a  current  of  1.2  to  1.3  amperes. t  The 
electrolysis  usually  requires  about  2  hours.  The  completeness  of 
the  deposition  may  be  tested  as  described  under  Copper  (p.  125) 
and  Lead  (p.  194)  by  diluting  with  a  little  water  and  observing 

*  For  practice,  0.2  to  0.3  gm.  of  metallic  antimony  is  pulverized  very  finely 
and  dissolved  by  heating  in  a  narrow  test  tube  with  about  1  cc.  of  concen- 
trated sulphuric  acid.  The  excess  of  acid  is  driven  off  and  the  cold  residue 
dissolved  in  a  saturated  solution  of  sodium  sulphide. 

t  If  the  antimony  is  present  in  the  form  of  a  precipitate  which  may  con- 
tain members  of  the  copper  group,  a  separation  from  the  latter  is  obtained  by 
warming  the  precipitate  with  sodium-sulphide  solution.  In  this  case,  there 
is  always  some  polysulphide  formed,  as  is  shown  by  the  yellow  color  of  the 
solution  and  before  adding  the  prescribed  quantity  of  potassium  cyanide, 
enough  of  this  reagent  should  be  added  to  decolorize  the  solution. 

t  The  potential  should  be  1.1  to  1.4  volts  and  must  not  exceed  1.7  volts. 
Periodic  changes  will  take  place  in  the  voltage  during  the  analysis.  K. 
Koelichen  studied  this  phenomenon  (Z.  Elektrochem.,  7,  629  (1901))  and  found 
it  due  to  the  alternating  deposition  and  solution  of  a  thin  layer  of  sulphur  on 
the  anode. 


158  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

after  about  10  minutes  whether  any  further  deposit  is  obtained  on 
the  freshly  exposed  surface. 

When  the  deposition  is  complete,  the  current  is  turned  off,  the 
liquid  poured  out  of  the  dish,  which  is  washed  with  water  and  with 
alcohol,  dried  at  80°  or  90°  in  an  air  bath  and  weighed  after  cooling 
in  a  desiccator  (cf.  p.  120) . 

The  antimony  may  be  easily  removed  from  the  dish  by  heating 
it  with  a  mixture  of  nitric  and  tartaric  acids. 

If  it  is  not  a  question  of  great  accuracy,  or  if  it  is  desired  to 
use  a  platinum  gauze  cathode,  the  antimony  sulphide  may  be 
dissolved  in  about  80  cc.  of  a  saturated  sodium-sulphide  solution, 
30  cc.  of  potassium-cyanide  solution  added,  as  above,  and  the  con- 
tents of  the  small  beaker  diluted  with  water  until  the  gauze  elec- 
trode is  completely  immersed.  The  rest  of  the  analysis  is  carried 
out  as  before. 

It  was  formerly  recommended  to  prepare  the  sodium-sulphide 
solution  in  the  laboratory  from  hydrogen  sulphide  and  pure 
sodium  hydroxide,  but  it  is  now  possible  to  buy  pure  sodium  sul- 
phide and  thus  the  somewhat  tedious  operation  may  be  avoided. 

If  the  antimony  solution  was  prepared  by  the  method  described 
on  page  157,  footnote  1,  it  may  contain  tin,  arsenic  and  traces  of 
copper.  The  fact  that  copper  has  no  disturbing  effect  has  been 
mentioned  (p.  155).  The  metkod  of  carrying  out  the  analysis 
when  tin  or  arsenic  is  present  will  be  given  under  the  separations. 

Tin. 

At.  Wt.  =  118.7.  Elec.  Equiv.  =  0.614  mg.  for  Sn  +  +  ions. 
Elec.  Potential  =  <  +  0.192  volt.  Overvoltage  of  H2  = 
0.43-0.53  volt. 

Tin  deposits  are  often  difficult  to  remove  completely  from 
the  electrode.  After  treatment  with  concentrated  hydrochloric 
acid,  a  dark  stain  (probably  of  Sn-Pt  alloy)  is  likely  to  remain. 
This  stain  can  be  removed  by  placing  the  electrode  in  molten 
potassium  pyrosulphate.  Henz  states  that  treatment  with  a 
mixture  of  nitric  and  oxalic  acids  is  a  more  rapid  means  of  dis- 
solving a  tin  deposit.  If  a  dark  residue  remains  it  may  be  removed 
by  Bunsen's  method,  which  consists  of  treating  with  zinc  and 
dilute  hydrochloric  acid  followed  by  concentrated  hydrochloric 
acid.  To  avoid  any  difficulty,  in  cleaning  the  electrode,  it  is 


TIN  159 

perhaps  best  to  deposit  a  thin  film  of  copper  on  the  platinum  before 
using  it  for  the  tin  determination. 

Tin  hydroxide  is  amphoteric.  In  very  dilute  solution,  par- 
ticularly when  in  the  quadrivalent  state,  there  is  a  marked  tend- 
ency toward  hydrolysis,  with  the  resulting  precipitation  of 
hydrated  tin  oxide,  unless  the  tin  is  in  the  form  of  a  complex 
ion.  From  nitric-acid  solutions,  the  precipitation  of  the  hydrated 
oxide  (metastannic  acid)  may  be  made  complete. 

There  are  two  methods  which  have  proved  satisfactory  for  the 
electrolytic  determination  of  tin;  the  electrolysis  of  a  solution 
containing  the  complex  ammonium  stannic  oxalate  and  the 
electrolysis  of  ammonium  thiostannate.*  The  latter  method  has 
proved  especially  advantageous  in  the  rapid  electro-analysis  as 
well  as  in  the  determination  of  tin  present  as  metastannic  acid 
and  contaminated  with  copper  or  other  metals  whose  sulphides 
are  insoluble  in  ammonium  sulphide  (cf.  Bronzes). 

Deposition  of  Tin  from  Acid-oxalate  Solution. 

Stannic  oxide  (also  the  sulphide)  as  obtained  in  the  course  of 
analytical  operations  (but  not  cassiterite)  may  be  dissolved  by 
heating  it  with  a  solution  of  ammonium  oxalate  or  of  acid  ammo- 
nium oxalate.  If  the  solution  of  the  normal  oxalate  is  subjected  to 
the  action  of  the  electric  current,  the  tin  is  at  first  deposited  upon 
the  cathode  in  a  bright  metallic  form  but  as  the  ammonium 
oxalate  is  gradually  oxidized  at  the  anode  to  ammonium  carbon- 
ate and  carbon  dioxide, 

(NH4)2C204  +  O  =  (NH4)2C03  +  C02, 

the  solution  becomes  alkaline  (owing  to  hydrolysis  of  ammonium 
carbonate)  and  stannic  acid  separates.  The  principal  condition 
for  the  quantitative  electrodeposition  of  tin  is,  therefore,  to  keep 
the  solution  acid  with  oxalic  acid  until  the  end. 

F.  Henz  f  found  it  better  to  liberate  the  oxalic  acid  from  the 
ammonium  oxalate  rather  than  to  add  fresh  oxalic  acid  from  time 
to  time.  The  sulphuric  acid  is  added  after  the  electrolysis  has 
been  in  progress  for  some  time,  when  some  of  the  ammonium  oxa- 
late has  undergone  the  above  reaction.,  Thus,  besides  setting 

*  Methods  of  A.  Classen:  Classen  and  v.  Reis,  Ber.,  14, 1622  (1881);  Classen, 
ibid.,  17,  2467  (1884);  18,  1104  (1885);  Bongartz  and  Classen,  ibid.,  21,  2900 
(1888);  Classen,  ibid.,  27,  2060  (1894). 

t  Z.  anorg.  Chem.,  37,  39  (1903). 


160          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

free  oxalic  acid,  more  or  less  ammonium  sulphate  is  formed  and 
this  salt  has  a  favorable  effect. 

To  carry  out  the  analysis,  the  solution  of  the  tin  salt  is  treated 
with  100  cc.  of  a  solution  containing  3.6  gms.  of  ammonium  oxa- 
late  and  an  equal  weight  of  free  oxalic  acid,  and  electrolyzed  with 
a  current  of  0.2  to  0.6  ampere  (corresponding  to  2.7  to  3.8  volts) 
at  the  room  temperature.  After  about  2  hours,  8  cc.  of  sulphuric 
acid  (1  :  1)  are  added.  The  precipitation  of  about  0.3  gms.  of 
tin  requires  from  8  to  10  hours.  The  tin  deposits  upon  the  elec- 
trode, which  has  been  previously  plated  with  copper,  in  the  form 
of  a  glistening,  metallic  layer  that  adheres  well.  Some  author- 
ities have  claimed  that  only  a  little  tin  will  adhere  to  the  cathode, 
but  Bongartz  and  Classen*  have  shown  that  deposits  weighing 
as  much  as  1  gram  can  be  obtained  satisfactorily.  After  break- 
ing the  circuit,  the  cathode  is  washed,  in  the  usual  manner  with 
water  and  alcohol,  and  dried  at  80°  to  90°. 

The  experiments  of  M.  Heidenreich  in  the  author's  labbratory 
have  shown  that  about  0.3  gm.  of  tin  may  be  deposited  in  4  to 
4.5  hours  by  a  current  of  NDioo  =  1  to  1.5  amperes,  if  the  elec- 
trolysis is  carried  out  at  a  temperature  of  60°  to  65°.  In  this 
case  the  washing  must  take  place  before  the  current  is  turned  off. 
Like  antimony,  tin  is  often  obtained  in  the  course  of  a  chemical 
analysis  as  dissolved  alkali  thio-salt.  To  change  such  a  solution 
into  one  of  oxalic  acid,  Henz  acidifies  it  with  acetic  acid  and 
then,  without  filtering  off  the  precipitated  stannic  sulphide,  adds 
a  boiling  hot  solution  of  ammonium  oxalate  and  oxalic  acid  (of 
the  concentration  stated  above),  using  100  cc.  for  each  0.1  gm.  of 
tin.  The  stannic  sulphide  dissolves  and  the  solution  is  turbid  only 
with  precipitated  sulphur  which  has  no  effect  upon  the  electrolysis. 

If  this  last  solution  is  electrolyzed,  after  cooling  to  room  temper- 
ature, with  a  current  of  NDioo  =  0.2  to  0.3  ampere  (corresponding 
to  2  or  3  volts),  the  greater  part  of  the  tin  will  be  deposited  in  6 
hours.  Then  8  cc.  of  sulphuric  acid  (1:1)  are  added  and  the 
electrolysis  is  continued.  After  24  hours  from  the  start,  all  the 
tin  will  have  been  deposited.  The  current  is  broken,  the  cathode 
washed  with  water  and  alcohol  and  dried  at  80-90°  before  weighing. 

If,  in  the  necessary  preparation  of  the  electrolyte,  the  volume 
of  the  tin  solution  becomes  too  large  to  be  contained  in  the  usual 
platinum  dish,  the  electrolysis  is  carried  out  in  a  beaker  and  a  gauze 
*  Ber.,  21,  2900  (1888). 


TIN  161 

cathode  is  used.  In  this  case,  when  the  electrolysis  is  finished, 
the  electrodes  are  quickly  raised  from  the  acid  solution  and  trans- 
ferred to  a  beaker  containing  distilled  water. 

If  the  electrolysis  is  carried  out  at  ordinary  temperatures,  the 
long  time  is  recommended  because  there  is  no  satisfactory  test  for 
traces  of  tin  in  oxalic-acid  solution  and  the  color  of  the  metal  is 
so  similar  to  that  of  platinum  that  it  is  hard  to  tell  whether  there 
is  any  slight  deposit  on  a  freshly  exposed  cathode  surface. 

If,  however,  the  tin  solution  is  heated  to  60°  at  the  start  and  the 
sulphuric  acid  is  added  after  3  hours,  one  may  feel  certain  after 
another  5  hours  that  as  much  as  0.2  gm.  of  tin  will  have  been 
precipitated  quantitatively.  It  is  necessary  to  keep  the  deposit 
thoroughly  wet  throughout  the  electrolysis,  adding  water  from 
time  to  time  to  replace  that  lost  by  evaporation. 

If  the  thiostannate  solution  is  one  from  which  antimony  has 
just  been  determined  by  electrolysis,  the  solution  will  contain 
hardly  any  polysulphide  because  the  chief  requirement  of  a  suc- 
cessful antimony  determination  is  the  absence  of  polysulphide. 
If,  however,  the  solution  is  yellow  colored,  it  is  heated  before  the 
addition  of  the  acetic  acid,  and  freshly  prepared  potassium  cyanide 
is  added,  drop  by  drop,  until  the  solution  becomes  colorless. 
Otherwise,  too  much  sulphur  is  precipitated  upon  the  addition 
of  acetic  acid,  and  it  is  hard  to  tell  whether  the  stannic  sulphide 
is  dissolved  completely  by  the  addition  of  ammonium  oxalate  and 
oxalic  acid. 

Rapid  Deposition  of  Tin  from  Ammonium-sulphide  Solution.* 

Experience  has  shown  that  the  electrolysis  of  an  ammonium 
thiostannate  solution  often  yields  spongy  deposits.  If  the  plat- 
inum cathode  is  given  a  thin  coating  of  copper  (see  Zinc)  and 
then  a  thin  coating  of  tin  (best  by  the  electrolysis  of  an  acid  solu- 
tion of  ammonium  stannic  oxalate,  p.  160)  the  deposit  of  tin  ob- 
tained from  the  thiostannate  solution  is  very  satisfactory.  This  is 
due  to  the  over  voltage  which  hydrogen  shows  toward  tin  (p.  82). 

*  General  reference?  to  the  rapid  electrodeposition  of  tin  in  various  solu- 
tions: Medway,  Am.  J.  Sci.,  [4],  18,  56,  180  (1904);  Z.  anorgan.  Chem.,  42, 
114  (1904);  Exner,  J.  Am.  Chem.  Soc.,  26,  896  (1903);  A.  Fischer,  Z.  anorgan. 
Chem.,  42,  382  (1904);  A.  Fischer  and  Boddaert,  Z.  Elektrochem.,  10,  945 
(1904);  L.  F.  Witmer,  J.  Am.  Chem.  Soc.,  29,  473  (1907);  Smith  and  Kollock, 
J.  Am.  Chem.  Soc.,  27,  1527  (1905). 


162          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 


Experiment  performed  by  A.  Fischer 
at  Aachen. 


Electrode 

Electrolyte  contained  . . . 

Volume 

Quantity  of  metal 

Temperature 

Voltage  of  the  bath 

Current  density 

Number  of  revolutions. . . 
Time  in  minutes .  . 


Gauze  electrode  and  lattice  stirrer 
16  cc.  CNHi)2S  *  and  20  cc.  Na2SO3  (40  per  cent) 

120  cc. 
0.2  gm.  as  SnCl4.2  NH4C1 

60° 

3.2  to  4  volts 

5.5  amperes 

About  800 

25 


The  metal  is  washed  with  alcohol,  then  with  carbon  disulphide 
and  finally  with  alcohol. 

The  rapid  electrodeposition  of  tin  with  the  aid  of  magnetic 
stirring  is  described  on  page  78. 

Arsenic. 

All  experiments  with  regard  to  the  electrolytic  determination 
of  arsenic  have  proved  unsatisfactory.  This  is  even  true  of  the 
method  proposed  by  B.  Neumann  f  which  consisted  in  electrolyz- 
ing  arsenious  acid  in  fuming  hydrochloric  acid  with  lead  or  silver 
anodes  and  a  potential  of  about  one  volt.  This  proves,  it  is  true, 
the  possibility  of  quantitatively  depositing  the  arsenic  as  metal 
on  the  cathode  but  the  experiment  is  such  a  tedious  one  that  it 
has  no  practical  significance. 

The  section  of  this  book  on  Metal  Separations  will  show  how 
arsenic  may  be  separated  from  other  elements  by  means  of  the 
electric  current. 

Recently,  experiments  have  been  made  which  show  that  the 
Marsh  test  can  be  carried  out  satisfactorily  for  forensic  purposes 
in  such  a  way  that  the  hydrogen  necessary  for  the  formation  of 
arsine  is  formed,  not  by  the  action  of  acid  upon  zinc  but  by  the 
electrolytic  decomposition  of  the  acid.  In  this  way  there  is  no 
danger  of  getting  a  test  from  the  arsenic  likely  to  be  present  in 
zinc.  Here,  it  will  suffice  to  refer  to  the  literature  on  the  subject.f 
.  *  The  ammonium-sulphide  solution  is  prepared  from  aqua  ammonia,  sp. 
gr.  0.91. 

t  Chem.-Ztg.,  30,  33  (1906). 

IT.  E.  Thorpe,  J.  Chem.  Soc.,  London,  83,  974  (1903);  H.  J.  S.  Sand, 
ibid.,  86,  1018  (1904);  S.  R.  Trotmann,  J.  Soc.  Chem.  Ind.,  23,  117  (1904). 
See  also  Treadwell-Hall,  "Quantitative  Analysis." 


TELLURIUM  163 

Tellurium. 
At.  Wt.  =  127.5.    Elec.  Equiv.  =  0.661  mg.  for  bivalent  Te. 

G.  Pellini  *  has  obtained  satisfactory  results  from  a  chloride 
solution  to  which  ammonium  tartrate  has  beeh  added. 

Procedure.  Weigh  0.1  to  0.02  gm.  of  the  dioxide  into  a  plati- 
num dish  which  has  been  sand-blasted  on  the  inside  and  dissolve 
the  sample  in  5  cc.  of  concentrated  hydrochloric  acid.  Dilute  the 
solution  with  100  to  125  cc.  of  a  cold,  saturated  solution  of  am- 
monium tartrate  in  water  and  then  add  pure  water  till  the  total 
volume  is  about  170  cc.  Heat  the  solution  to  60°  and  electrolyze, 
using  the  dish  as  cathode,  with  a  current  of  NDioo  =  0.02  to  0.01 
ampere  and  1.85  to  2.2  volts  e.m.f.  When,  at  the  end  of  about  9 
hours,  a  portion  of  the  solution  gives  no  brown  precipitate  of 
tellurium  upon  treatment  with  stannous  chloride  and  hydrochloric 
acid,  wash  the  deposit  with  water  which  has  been  boiled  to  remove 
dissolved  oxygen  and  cooled  in  a  current  of  carbon  dioxide.  Finally 
rinse  with  alcohol  and  dry  at  90°  for  a  few  minutes. 

Rapid  Deposition  of  Tellurium. 

Larger  quantities  of  tellurium  (0.06  to  1.2  gms.)  were  determined 
by  the  same  authoi  IL.  well-stirred  electrolytes.  Starting  with 
metallic  tellurium,  the  solution  obtained  by  oxidation  with  nitric 
acid  was  evaporated,  the  tellurous  acid  dissolved  on  the  water 
bath  in  10  cc.  of  sulphuric  acid,  and  30  to  40  cc.  of  a  saturated 
solution  of  ammonium-acid-tartrate  added.  The  tellurous-acid 
solution  was  further  diluted  with  the  same  solution  to  250  cc. 
and  after  heating  to  60°  the  electrolysis  was  carried  out  with  a 
current  of  NDioo  =  0.12  to  0.09  ampere  at  1.8  to  1.2  volts.  The 
cathode  used  by  the  author  was  a  platinum  cylinder  roughened 
on  the  inside.  The  stirrer  was  made  to  revolve  from  800  to  900 
times  per  minute.  The  deposited  tellurium  was  washed  and 
dried  as  described  above. 

Gallo  f  found  that  the  above  method  was  not  wholly  satis- 
factory for  relatively  large  quantities  of  tellurium.  He  recom- 
mended the  following  procedure. 

*  Gazz.  chim.  ital.,  34, 1.  128  (1904). 

t  Atti  d.  Reale  Accad.  del  Lincei,  Roma  [5],  13, 1,  713  (1904). 


164          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Dissolve  the  metal  in  concentrated  sulphuric  acid,  using  a 
platinum  dish  which  has  been  roughened  on  the  inside  by  sand 
blasting.  Evaporate  till  fumes  of  sulphuric  acid  are  evolved, 
and  but  little  free  acid  remains.  Cool,  add  a  few  cubic  centi- 
meters of  boiled  water,  free  from  carbonic  acid,  and  then  dilute 
to  150  cc.  with  10  per  cent  sodium  pyrophosphate  solution. 
Electrolyze  with  a  current  of  NDioo  =  1.8  to  2  volts  at  1.8  to  2 
volts.  About  25  mgms.  of  metallic  tellurium  will  be  deposited 
per  hour. 


ZINC  165 


GROUP  II. 

This  group  includes  the  metals  indium,  cadmium,  and  zinc. 
The  exact  position  of  indium  in  the  voltage  series  is  in  doubt. 
The  metals  cadmium  and  zinc  are  considerably  above  hydrogen 
in  the  series  but,  thanks  to  the  overvoltage  which  they  show 
toward  the  evolution  of  hydrogen,  it  is  possible  to  deposit  them 
from  slightly  acid  solutions.  The  elements  will  be  discussed  in 
the  order  of  their  importance. 

Zinc. 

At.  Wt.  =  65.37.    Elec.  Equiv.  =  0.339  mg.  for  Zn  +  +  ions.  Elec. 
Potential  =  +  0.770  volt.     Overvoltage  of  H2  =  0.70  volt. 

Zinc  may  be  deposited  quantitatively  either  from  acid  or  from 
alkaline  solutions.  Many  methods  have  been  proposed  of  which 
only  a  few  of  the  best  will  be  mentioned. 

On  account  of  slight  oxidation,  the  values  obtained  by  all 
electrolytic  methods  for  the  determination  of  zinc  are  likely 
to  be  a  little  high  unless  special  pains  are  taken  to  prevent  such 
oxidation. 

Deposition  of  Zinc  from  Alkaline  Solutions. 

Beilstein  and  Jawein  in  1879  successfully  deposited  zinc  from  a 
potassium-cyanide  solution.  This  method  is  seldom  used  to-day 
because  it  is  simpler  and  quicker  to  use  an  alkaline  solution  with- 
out any  potassium  cyanide.*  G.  Vortmannf  added  alkali  tartrate 
to  the  alkaline  solution  and  stated  that  the  deposits  adhered  well 
irrespective  of  whether  little  or  much  caustic-soda  solution  were 
present.  After  R.  Amberg,|  on  the  basis  of  work  carried  out  in 
the  author's  laboratory  by  Millot  and  v.  Foregger,  had  found  that 
the  electrolytic  deposition  of  zinc  was  possible  from  an  alkaline 
solution  without  the  addition  of  any  other  chemical,  F.  Spitzer§ 

*  A  modification  of  the  potassium-cyanide  method  used  for  the  electrolytic 
separation  of  zinc  from  iron  will  be  given  in  Part  III. 
t  Monatsh.  Chem.,  14,  536  (1903). 
t  Ber.,  36,  2489  (1903). 
§  Z.  Elektrochem.,  11,  391  (1905). 


166 


QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 


succeeded  in  simplifying  the  method  by  proving  that  the  large 
excess  of  caustic  alkali  recommended  by  Amberg  was  unnecessary. 
Amberg  had  recommended  that  not  less  than  40  gms.  of  potassium 
hydroxide  should  be  used  for  0.5  gm.  of  zinc  but  Spitzer  found 
that  accurate  results  were  obtained  if  the  quantity  of  alkali  added 
was  large  enough  to  keep  the  solution  clear  during  the  progress 
of  the  analysis,  and,  to  accomplish  this,  at  least  10  molecules  of 
NaOH  are  necessary  for  1  molecule  of  ZnSO4. 

Spitzer's  directions,  therefore,  were  to  add  enough  sodium 
hydroxide  to  the  zinc-sulphate  solution  *  to  make  a  permanently 
clear  solution.  It  is  not  necessary  to  measure  the  alkali  very  care- 
fully for  Amberg's  work  has  shown  that  an  excess  of  this  reagent 
does  no  harm.  A  convenient  strength  of  the  caustic  alkali  is 
160  gms.  NaOH  in  a  liter  (4-normal).  The  quantity  of  zinc  used 
in  an  analysis  may  be  from  0.16  to  0.32  gm.  As  cathode  a 

RAPID    DEPOSITION    OF    ZINC    FROM    ALKALINE    SOLUTION. 


Experiments  performed  by 

A.  Fischer. 

Exner.                Ingham. 

Electrode 

Dish 
and  ro- 
tating 
disk 

20  gm. 
KOH 

125  cc. 

0.23 
gm.  as 
sul- 
phate 

95° 

3  volts 
600-800 
15  min. 

Gauze 
lat 

Enoi 
forn 

100  cc. 

0.2 
gm.  as 
sul- 
phate 

Cold 
durinj 
the 
r 

4  volts 

800- 
1000 

30  min. 

electro 
tice  stir 

igh  NaC 
i  the  zin 

100  cc. 

0.4 
gm.  as 
sul- 
phate 

at  the  s 
y  the  an 
jempera 
ises  to  6 

3.9  to  4 
volts 

800- 
1000 

20-15 
min. 

de  and 
rer 

Hto 

cate 

100  cc. 

0.4 
gm.  as 
sul- 
phate 

tart; 
alysis 
ture 
0° 

4.1 

volts 

800- 
1000 

5  min. 

Dish 

5  to  12 
gms. 
NaOH 

125  cc. 

0.5 
gm. 

Hot 

5  to  6 
volts 

600-800 
15  min. 

and  rot 
spiral 

6  gms. 
NaOH 

ating 

6  gms. 
NaOH 

Electrolyte  con- 
tained 

Volume 

Quantity  of  metal.  . 
Temperature 

0.25 
gm. 

Hot 

8  volts 
230 
20  min. 

0.48 
gm. 

Hot 

6  volts 
230 
30  min. 

Potential 

Revolutions  

Time  

1  For  practice,  zinc  vitriol,  ZnSO4.7H2O,  may  be  used. 


ZINC  167 

Winkler's  platinum  gauze  electrode,  which  is  given  a  preliminary 
coating  of  silver,  is  used  (see  below).  With  such  an  electrode 
about  0.3  gin.  of  zinc  can  be  deposited  quantitatively  at  ordinary 
temperatures  with  a  current  of  0.8  ampere  in  2  hours.  The 
current  may  be  reduced  to  0.3  ampere  if  it  is  not  desired  to  carry 
out  the  analysis  quickly.  The  potential  difference  between  the 
electrodes  with  the  above  current  is  about  4  volts.  The  deposit 
may  be  washed  after  turning  off  the  current  and  the  same 
electrode,  with  its  zinc  deposit  on  it,  may  be  used  for  several 
analyses.  After  washing  with  alcohol,  the  electrode  is  dried  at 
70°  to  80°. 

In  technical  laboratories  electrodes  of  amalgamated  brass 
gauze  have  been  used  on  account  of  their  cheapness  (cf. 
p.  178). 

The  deposition  of  zinc  from  ammoniacal  and  from  alkaline 
tartrate  solutions  will  be  described  under  the  separation  of  nickel 
from  zinc. 

The  above  table  shows  that  zinc  can  be  determined  rapidly 
from  alkaline  solutions  even  when  the  conditions  are  varied 
considerably.* 

Spear  and  Strahan  f  recommend  the  determination  of  zinc  from 
alkaline  solutions  but  have  obtained  the  best  results  under  condi- 
tions somewhat  different  from  those  given  above.  Their  method 
is  as  follows:  About  0.4  gm.  of  zinc,  present  as  sulphate,  is  treated 
with  an  aqueous  solution  of  10  to  25  gms.  KOH  and  the  total 
volume  of  the  electrolyte  made  up  to  125  cc.  The  solution  is 
brought  nearly  to  boiling  and  electrolyzed  with  a  current  of 
NDioo  =  3  amperes.  As  anode  a  platinum  spiral  is  used  and  it  is 
placed  above  (not  at  the  side)  the  rotating  cathode.  The  latter 
is  preferably  made  of  nickel  gauze,  30  meshes  to  the  inch,  which 
is  bent  over  (dome  shaped)  at  the  top  but  does  not  extend  to 
the  stout  wire  stem  of  the  same  metal.  Seven  to  eight  minutes 
before  the  end  of  the  experiment,  which  should  require  from  30 

*  General  references  on  the  rapid  electrolytic  determination  of  zinc  from 
various  solutions:  Medway,  Am.  J.  Sci.,  [4],  18,  56  (1904);  Z.  anorg.  Chem.,  42, 
114  (1904).  Exner,  J.  Am.  Chem.  Soc.,  25,  896  (1903).  Exner,  ibid.,  26, 
1269  (1904).  A.  Fischer  and  Boddaert,  Z.  Elektrochem.,  10,  945  (1904).  Per- 
kin,  Chem.  News,  93,  283  (1906).  Price  and  Judge,  ibid.,  94,  18  (1906).  A. 
Fischer,  Chem.-Ztg.,  31,  25  (1907).  E.  F.  Smith  and  Kollock,  J.  Am.  Chem, 
Soc.,  27,  1255  (1905).  H.  J.  S.  Sand,  J.  Chem.  Soc.,  London,  91,  373  (1907). 

t  J.  Ind.  Eng.  Chem.,  4,  889  (1912). 


168  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

to  45  minutes,  the  anode  and  sides  of  the  beaker  are  washed  down 
with  a  little  water.  Then,  after  electrolyzing  a  minute  longer, 
the  solution  is  cooled  to  below  25°,  using  ice  if  necessary.  When 
the  solution  is  cold,  the  beaker  is  lowered  without  interrupting 
the  current  and,  before  stopping  the  stirrer,  the  cathode  is  care- 
fully washed  with  water,  then  with  alcohol,  and  finally  with  ether 
that  has  been  recently  dried  over  sodium  and  freshly  distilled.  The 
ether  is  at  once  removed  by  gentle  heating  and  the  electrode 
weighed  after  standing  a  short  time  in  a  desiccator. 

Spear  and  Strahan  obtained  excellent  results  by  this  method. 
Nitrates  and  ammonium  salts  must  be  absent  or  the  deposition  of 
the  zinc  will  be  incomplete.  The  original  solution  should  not 
contain  much  free  sulphuric  acid,  as  zinc-potassium  sulphate  is 
not  very  soluble  in  concentrated  alkali  solutions.  High  results 
are  often  due  to  the  formation  of  hydroxide  on  the  cathode. 
This  may  be  due  to  exposure  of  the  cathode  to  the  air  or  to  the 
gases  arising  from  the  anode,  to  stopping  the  electrolysis  while 
the  solution  is  still  warm,  to  incomplete  removal  of  caustic  alkali 
before  washing  with  alcohol,  to  the  use  of  ether  containing  water 
and  oxides  that  attack  zinc,  or  to  letting  the  ether  evaporate 
spontaneously  in  a  desiccator.  There  is  always  a  weighable 
amount  of  zinc  hydroxide  formed  but  this  positive  error  is  almost 
exactly  compensated  in  the  above  method  by  slight  zinc  losses 
which  take  place  during  the  washing  with  water. 

Rapid  Deposition  of  Zinc  in  Ammoniacal  Solution. 

L.  H.  Ingham  *  has  found  that  the  deposition  of  zinc  from  an 
ammoniacal  solution,  which  is  successful  with  stationary  elec- 
trolytes only  under  special  conditions,  gives  good  results  if  the 
electrolyte  is  kept  in  motion.  The  presence  of  ammonium  chloride 
in  the  solution  has  a  favorable  effect,  rather  than  otherwise, 
because  it  serves  to  increase  the  conductivity  of  the  electrolyte. 
As  cathode  a  silvered  platinum  dish  and  as  anode  a  platinum 
spiral  of  about  50  mm.  diameter  is  used.  The  latter  is  arched 
slightly  to  make  it  correspond  to  the  surface  of  the  stirred  liquid 
and  is  made  to  revolve  about  230  times  in  a  minute.  From  a  solu- 
tion containing  0.24  gm.  of  zinc  sulphate,  with  5  cc.  of  hydrochloric 
acid  (sp.  gr.  1.21)  which  is  neutralized  with  ammonia  (sp.  gr.  0.95), 

*  J.  Am.  Chem.  Soc.,  26,  1280  (1904). 


ZINC  169 

and  the  solution  treated  with  one  gram  of  ammonium  chloride  in 
addition,  the  zinc  is  quantitatively  deposited  upon  the  cathode 
in  20  minutes  with  a  current  of  5  amperes  and  5  volts.  There  is 
no  injurious  effect  of  the  chlorine  at  the  anode.  This  method 
has  given  good  results  in  the  analysis  of  zinc-sulphide  ores  (see 
Part  IV). 

Deposition  of  Zinc  from  Acid  Solution. 

Before  the  significance  of  the  overvoltage  of  hydrogen  was 
recognized,  it  was  regarded  as  futile  to  attempt  the  electrolysis 
of  zinc  in  an  acid  solution  (cf.  p.  174).  Although  it  is  possible  to 
deposit  zinc  electrolytically  from  a  fairly  acid  solution  upon  a  zinc 
cathode,  and  such  an  electrode  exists  as  soon  as  a  layer  of  zinc  has 
been  formed  upon  the  platinum,  yet  the  reaction  comes  to  an  end 
as  soon  as  the  concentration  of  zinc  ions  has  become  diminished, 
while  that  of  hydrogen  ions  is  increased  to  the  point  where  less 
work  is  needed  by  the  current  to  discharge  the  hydrogen  ions 
than  to  discharge  the  zinc  ions.  To  counteract  this  tendency, 
the  concentration  of  free  acid  must  be  kept  very  low,  by  using 
an  acid  such  as  acetic,  tartaric,  formic,  etc.,  and  by  adding  a 
salt  of  the  same  acid  to  the  electrolyte  bath.  In  this  way  the 
deposition  of  zinc  can  be  made  quantitative. 

(a)  The  solution  contains  sodium  acetate  and  acetic  acid.  The 
electrolysis  of  zinc  solutions  containing  these  substances  was 
recommended  by  Riche,  Parodi  and  Mascazzini,  as  well  as  by 
Rudorff  and  has  been  tested  by  F.  Spitzer.*  By  using  a  gauze, 
electrode,  good  deposits  are  obtained  with  a  current  strength  of 
0.5  ampere  if  the  solution  containing  about  0.16  gm.  of  zinc  in 
100  cc.  is  treated  with  a  solution  of  5  gms.  sodium  acetate  and 
acidified  with  0.3,  or  0.5  cc.  at  the  most,  of  glacial  acetic  acid. 
The  analysis  is  carried  out  at  the  ordinary  temperature  and  re- 
quires from  2  to  2.5  hours. 

Too  high  a  current  density  or  too  much  acetic  acid  causes  an 
uneven  deposit;  a  branching,  crystalline  growth  is  obtained  which 
easily  falls  off  the  electrode.  To  make  sure  that  the  acidity  of 
the  solution  is  not  too  great,  any  free  acid  originally  present  in 
the  solution  is  neutralized  by  the  cautious  addition  of  caustic-soda 
solution,  before  adding  the  sodium  acetate. 

*  Z.  Elektrochem.,  11,  404  (1905). 


170  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Rapid  Deposition  of  Zinc  from  Acetic-acid  Solutions. 

It  is  technically  very  important  to  have  a  rapid  and  reliable 
method  for  the  determination  of  zinc.  The  extent  to  which  this 
problem  has  occupied  the  attention  of  different  investigators  is 
shown  by  the  monograph  of  H.  Nissenson.*  This  author  believes 
that  the  rapid  electrolytic  methods  for  determining  zinc  will 
eventually  replace  the  volumetric  methods  generally  used  where 
a  great  many  zinc  determinations  have  to  be  made.  The  chief 
advantage  of  the  volumetric  methods  lies  in  the  rapidity  at  which 
the  analyses  are  carried  out  but  the  objection  to  them  is  that  the 
end  point  depends  upon  an  accurate  judgment  of  a  color  shade, 
so  that  an  operator  can  be  certain  of  his  results  only  when  he  has 
had  constant  and  recent  practice  with  the  determination.  The 
results  obtained  by  H.  J.  Sand,  which  are  taken  from  the  paper 
referred  to  on  page  42,  will  be  given  here  in  detail. 

All  the  electrolytes  used  by  this  investigator  contained  a  little 
free  acetic  acid  and  also  some  alkali  sulphate,  because  thfs  salt  is 
almost  always  present  in  the  analysis  of  zinc  ores.  The  excess  of 
sulphuric  acid  was  neutralized  in  some  cases  by  the  addition  of  a 
large  excess  of  ammonia  and  in  other  cases  by  sodium  hydroxide; 
but,  at  the  last,  enough  acetic  acid  was  always  added  to  make  the 
solution  slightly  acid.  As  it  has  been  shown  that  it  is  hard  to 
deposit  the  last  traces  of  zinc  when  the  electrolyte  is  at  a  temper- 
ature above  30°,  it  was  necessary  to  keep  the  electrolytic  cell  sur- 
rqunded  by  cold  water  in  order  to  prevent  the  solution  becoming 
heated  by  the  passage  of  the  current. 

All  these  experiments  were  carried  out  with  Sand's  electrodes 
(p,  66)  and  the  cathode  was  first  covered  with  a  copper  de- 
posit. 

ly.  experiments  2,  3  and  4  the  auxiliary  electrode  was  used 
(cf.  p.  40).  The  volume  of  the  electrolyte  was  about  85  cc.  in  all 
cases  and  the  stirrer  made  between  300  and  800  revolutions  per 
minute. 

In  experiments  4,  5  and  7  the  first  values  for  zinc  were  obtained 
after  20  minutes  and  the  second  values  were  obtained  by  con- 
tinuing the  electrolysis  until  the  weight  of  the  cathode  became 
constant,  which  required  10  minutes  more.  It  will  be  noted 
that  most  of  the  results  are  a  little  too  high  (cf.  p.  168). 

*  Die  Bestimmungsmethoden  des  Zinks,  Stuttgart,  1907. 


ZINC 


171 


o 

B 

§ 


<3  3 

a  '.S 

|  a 

CO  CO 


n 


20+  10  minu 


d   do   ****** 


III  r! 


o      o 


<s 


° 


8 


=3 


0 


1  1 


172 


QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 


RAPID    DEPOSITION    OF    ZINC    FROM    ACETIC-ACID    SOLUTION. 


Experiments  performed  by 

A.  Fischer. 

Exner. 

Ingham. 

A.  Fischer. 

Electrode 

Dish  and 

Dish  and 

Dish  and 

Gauze  elec- 

rotating 

rotating 

rotating 

trodes  and 

disk 

spiral 

spiral 

lattice 

stirrer 

Electrolyte  contained. 

1  to  3  gms. 
of  sodium 

1  to  3  gms. 
of  sodium 

3  gms.  so- 
dium ace- 

1.25 cc.  of 
cone.  H2SO4, 

acetate  and 

acetate  and 

tate  and 

3.5  cc.   of 

0.2%  of 

0.2%  of 

4  to  6  cc.  of 

cone. 

acetic  acid 

acetic  acid 

acetic  acid 

NH4OH, 

(30%) 

(30%) 

(30%) 

1.5  cc.  of 

glacial 
HC2H302, 

2.25  gms.  of 

NH4C2H3O2 

Volume 

125  cc. 

125  cc. 

125  cc. 

100  cc. 

Quantity  of  metal  

0.25  to  0.5 

gm.  as  sul- 

0.25 to  0.5 
gm.  as  sul- 

0.25 to  0.5 
gm.  as  sul- 

0.4 gm.  as 
sulphate 

phate 

phate 

phate 

Temperature 

20° 

Hot 

Hot 

Under  30° 

Potential  

6.5  to  8.7 

10  to  8 

12  to  17 

3.9  to  4.2 

volts 

volts 

volts 

volts 

Revolutions     .    . 

600  to  800 

600  to  800 

230  to  560 

1200 

Time  

15  min. 

10  to  15  min. 

10  to  15  min. 

30  min. 

(b)  The  solution  contains  ammonium  oxalate  and  free  oxalic  acid 
or  tartaric  acid.*  The  zinc  salt  is  dissolved  by  warming  with  as 
little  water  as  possible,  about  4  gms.  of  ammonium  oxalate  are 
added  and  complete  solution  is  effected  by  heating,  adding  a  little 
more  water  if  necessary,  f  The  clear  solution  is  transferred  to  a 
platinum  dish,  which  has  been  coated  with  either  copper  or  silver 
(see  below),  diluted  to  120  cc.,  heated  to  50°  or  60°  and  electro- 
lyzed  with  a  current  of  0.5  to  1  ampere  and  3.5  to  4.8  volts. 

The  author  has  found  by  experiment  that  a  dense,  metallic 
deposit  is  formed  only  by  keeping  the  solution  acid  during  the 
electrolysis.  For  acidifying,  a  cold  saturated  solution  of  oxalic 
acid  or,  better,  a  solution  of  3  gms.  of  tartaric  acid  in  50  cc.  of 

*  The  method  has  been  credited  to  Reinhardt  and  Ihle,  but  was  previously 
used  by  the  author;  cf.  Classen  and  v.  Reis,  Ber.,  14,  1662  (1881)  and  Classen, 
ibid.,  27,  2060  (1894). 

t  The  addition  of  ammonium  oxalate  to  a  dilute  solution  of  zinc  salt 
causes  a  precipitate  of  zinc  oxalate  to  form  which  is  not  entirely  converted  to 
ammonium  zinc  oxalate  by  a  dilute  solution  of  ammonium  oxalate. 


ZINC  173 

water,*  is  employed.  After  electrolyzing  for  from  3  to  5  minutes, 
the  acid  solution  is  permitted  to  flow  in  drops  (about  ten  to  the 
minute),  from  a  burette  with  a  fine  outlet,  upon  a  watch  glass 
covering  the  dish.  The  acid  runs  slowly  into  the  solution  through 
a  perforation  in  the  watch  glass. 

The  reduction  requires  about  2  hours.  When  all  the  zinc  has 
been  deposited,  as  shown  by  warming  a  little  of  the  solution 
with  potassium  ferrocyanide  and  finding  no  brownish  precipitate 
formed,  the  deposit  is  washed  with  water  without  breaking  the 
circuit,  rinsed  with  alcohol,  and  dried  at  70°  to  80°  in  an  air  bath. 

If  the  zinc  is  deposited  directly  upon  a  platinum  surface,  then 
on  dissolving  the  deposit  in  dilute  hydrochloric  or  sulphuric  acid, 
a  dark  coating  of  platinum  black  usually  remains  which  can  be 
removed  best  by  igniting  the  dish  and  again  treating  it  with  acid. 
Since  the  dish  is  appreciably  attacked  by  this  operation,  it  is 
advisable,  before  weighing  the  dish,  to  precipitate  upon  it  a  thin 
coating  of  copper  or,  better,  silver. 

A  bright,  thick  coating  of  copper  can  be  obtained  in  a  few 
minutes  if  a  saturated  solution  of  copper  sulphate  is  treated  with 
an  excess  of  ammonium  oxalate  to  form  the  double  salt,  acidified 
with  oxalic  acid,  warmed  to  70°  to  80°,  and  the  copper  precipitated 
by  a  current  of  1  ampere.  The  preparation  of  the  double  salt  in 
a  beaker  and  the  transfer  of  the  clear  hot  solution  to  the  platinum 
dish  is  to  be  recommended. 

For  silvering  the  dish  it  is  best  to  precipitate  the  silver  from  a 
solution  containing  potassium  cyanide  (see  p.  133). 

Von  Miller  and  Kiliani  have  modified  the  oxalate  method  as 
follows.  In  the  copper-plated  platinum  dish,  4  gms.  of  potassium 
oxalate  and  3  gms.  of  potassium  sulphate  are  dissolved  and  to  this 
aqueous  solution  (not  the  reverse)  the  zinc  nitrate  or  zinc  sulphate 
(but  not  chloride)  solution  is  added  after  it  has  been  carefully  neu- 
tralized with  caustic  potash.  The  mixture  is  electrolyzed  at  the 
room  temperature  with  a  current  of  NDioo  =  0.3  ampere.  The 
electrolysis  requires  from  2  to  3  hours.  By  stirring  the  electrolyte, 
the  current  density  may  be  increased  to  0.5  ampere  and  the  time 
diminished. 

The  solution  must  not  contain  any  ammonium  salts  or  chloride. 
The  latter,  especially,  causes  a  spongy  deposit  to  be  obtained. 

*  Nicholson  and  Avery,  Z.  Elektrochem.,  3,  150  (1896),  prefer  to  acidify 
with  formic  acid. 


174          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Cadmium. 

At.  Wt.  =  112.4.    Elec.  Equiv.  =  0.582  mg.  for  Cd  +  +  ions.   Elec. 
Potential  =  +  0.770  volt.     Overvoltage  of  H2  =  0.70  volt. 

Deposition  from  Sulphuric-acid  Solution. 

The  electrolytic  deposition  of  cadmium  from  a  solution  con- 
taining a  considerable  quantity  of  free  sulphuric  acid  would  at  one 
time  have  been  regarded  as  theoretically  impossible.  Cadmium, 
like  zinc,  belongs  to  the  class  of  metals  of  which  the  discharge 
potential  is  greater  than  that  of  hydrogen  ions  (cf.  p.  81),  thus, 
in  an  acid  solution,  hydrogen  should  be  liberated  before  the  cad- 
mium ions  are  discharged,  or,  in  a  weakly  acid  solution,  there 
might  be  a  simultaneous  discharge  of  cadmium  and  hydrogen  ions. 
Owing  to  the  bad  effect  of  hydrogen  upon  the  nature  of  the  deposit 
(cf.  p.  22),  there  would  seem  to  be  little  prospect  of  obtaining 
a  satisfactory  deposit  of  cadmium  by  the  electrolysis  of  an  acid 
solution. 

The  placing  of  cadmium  among  the  metals  which  are  positive 
with  respect  to  hydrogen  is  justifiable,  or,  in  other  words,  the 
discharge  potential  of  cadmium  is  higher  than  the  potential  of 
hydrogen,  only  when  the  hydrogen  or  cadmium  ions  are  in  contact 
with  a  platinum  cathode.  After  Nernst  and  Caspari*  showed 
that  it  requires  a  higher  electromotive  force  to  discharge  hydrogen 
ions  in  contact  with  a  metal  other  than  platinum  (for  example, 
cadmium)  the  possibility  of  cadmium  ions  being  discharged  before 
hydrogen  ions  became  apparent  and  experiments  with  cadmium 
in  weakly  acid  sulphate  solutions  were  renewed.  Even  before 
this  was  understood,  Luckow  succeeded  in  depositing  cadmium 
quantitatively  from  a  dilute  sulphuric-acid  solution.  Since  the 
discovery  of  the  overpotential  of  hydrogen  towards  metals  other 
than  platinum,  the  cadmium  deposition  has  been  explained  very 
easily.  At  the  beginning  of  the  electrolysis,  the  concentration  of 
cadmium  ions  is  sufficient  to  cause  deposition  of  cadmium  upon 
the  platinum  electrode  even  though  some  hydrogen  may  be 
evolved  at  the  same  time,  and  when  this  layer  of  cadmium  has 
formed  upon  the  surface  of  the  platinum,  the  overvoltage  of 
hydrogen  is  such  that  from  now  on  the  cadmium  will  deposit 
more  readily  than  hydrogen  ions  will  be  discharged.  Bala- 
chowsky  has  obtained  good  results  using  as  cathode  a  platinum 
*  Z.  physik.  Chem.,  30,  89  (1899). 


CADMIUM  175 

dish  which  was  first  plated  with  silver  or  copper  before  beginning 
the  electrolysis  of  the  cadmium  solution.  For  the  same  reason, 
Hollard  and  Bertiaux  recommended  the  formation  of  a  preliminary 
deposit  of  cadmium. 

P.  Denso*  succeeded  in  obtaining  a  satisfactory  deposition  of 
cadmium  by  using  gauze  cathodes  without  any  preliminary  prep- 
aration. The  cadmium-sulphate  solution  wa.  0.05  normal  in  re- 
spect to  free  sulphuric  acid  (about  0.25  per  cent  H2SO4)  and  was 
electrolyzed  overnight  with  a  current  of  0.045  to  0.25  ampere  at 
a  potential  of  2.6  to  3.3  volts.  In  this  way  0.21  gm.  of  cadmium 
was  obtained  in  a  weighable  form. 

In  a  solution  which  was  normal  in  respect  to  free  sulphuric  acid, 
it  was  possible  to  deposit  0.1  gm.  of  cadmium  in  3  hours  with  a 
current  of  0.16  ampere  at  2.6  volts.  The  same  quantity  of  metal 
in  a  double-normal  acid  solution  (about  10  per  cent  free  B^SOJ 
required  5  hours  with  a  current  of  0.29  ampere  at  2.7  volts. 

The  deposit  of  cadmium  must  be  washed  with  water  before 
breaking  the  circuit. 

Hollard, f  realizing  the  significance  of  the  overpotential  of 
hydrogen  with  respect  to  the  deposition  of  metal,  covers  the 
platinum  gauze  electrode  with  a  layer  of  cadmium  before  begin- 
ning the  analysis.  This  is  done  most  conveniently  by  electrolyz- 
ing  with  a  current  of  0.4  ampere  a  cadmium-sulphate  solution  to 
which  8  gms.  of  potassium  cyanide  and  4  gms.  of  sodium  hydroxide 
have  been  added.  In  this  way  a  beautiful,  silver-white  coating  of 
cadmium  is  obtained.  After  washing,  drying  and  weighing  the 
electrode,  it  possesses  the  requisite  surface  to  obtain  a  quantita- 
tive deposit  of  metal  from  very  dilute  cadmium  solutions. 

The  solution  of  cadmium  sulphate  is  treated  with  5  cc.  of  con- 
centrated sulphuric  acid,  a  solution  containing  10  gms.  of  sodium 
sulphate  is  added,  and,  after  diluting  to  a  volume  of  300  cc.,  the 
electrolysis  is  carried  out  with  a  current  of  1  ampere. 

If  more  than  0.1  gm.  of  cadmium  is  present  in  300  cc.  of  solution 
the  preliminary  plating  of  the  cathode  with  a  layer  of  cadmium 
is  unnecessary  because  in  such  cases  the  original  concentration  of 
cadmium  ions  is  sufficient  to  prevent  any  injurious  effect  of  the 
hydrogen  ions  (cf.  p.  174). 

Holland  explains  the  favorable  effect  obtained  by  adding  the 
sodium  sulphate.  He  assumes  that  the  hydrogen  ions  form  a  com- 

*  Z.  Elektrochem.,  9,  468  (1903).         f  Bull.  soc.  chim.,  29,  217  (1903). 


176  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

plex  compound  with  the  salt  and  are  rendered  less  harmful  thereby. 
The  result  of  conductivity  measurements  confirms  the  assumption 
that  a  complex  compound  is  formed.  Hollard  found  that  the  sum 
of  the  conductivities  of  the  sodium-sulphate  solution  and  of  the  sul- 
phuric acid,  when  measured  separately,  was  greater  than  the 
conductivity  of  the  solution  containing  the  mixture  of  sodium 
sulphate  and  sulphuric  acid.  The  difference  was  more  than  could 
be  accounted  for  by  the  mutual  effect  of  sodium  sulphate  and 
of  sulphuric  acid  upon  the  electrolytic  dissociation  of  the  original 
molecules. 

The  electrodeposition  of  cadmium  from  sulphuric-acid  solutions 
serves  to  afford  a  separation  of  this  metal  from  zinc  and  from  iron, 
as  will  be  shown  later. 

Deposition  of  Cadmium  from  Alkaline  Cyanide  Solutions. 

According  to  the  present  status  of  electro-analytical  practice, 
the  method  first  recommended  by  Beilstein  and  Jawein*  and 
subsequently  improved  by  E.  Rimbachf  for  depositing  cadmium 
from  the  solution  of  the  complex  cyanide  is  quite  generally  pre- 
ferred. The  metal  obtained  from  such  a  solution  has  a  silver- 
white  color,  whereas  it  is  more  grayish  when  obtained  otherwise, 
and  the  cathode  never  requires  any  special  preparation,  as  in  the 
methods  just  described.  The  disadvantages  lie  in  the  general 
unpleasantness  of  working  with  cyanide  solutions  when  other 
determinations  are  to  be  carried  out  in  the  solution  after  the 
removal  of  the  cadmium,  and  the  further  fact  that  the  electrolysis 
requires  a  comparatively  long  time. 

If  the  time  factor  is  not  important,  and  if  it  is  not  going  to  be 
necessary  to  remove  the  excess  of  potassium  cyanide  from  the 
electrolyzed  solution,  the  electrolysis  may  be  carried  out  over- 
night and  in  the  following  manner: 

The  solution  of  cadmium  sulphate,  or  chloride,  is  treated,  in 
case  it  contains  free  acid,  with  enough  caustic-potash  solution  to 
neutralize  all  the  acid  and  then  a  solution  of  pure  potassium  cyanide 
is  added  until  the  precipitate,  which  is  first  formed,  disappears 
entirely.  A  large  excess  of  potassium  cyanide  should  be  avoided. 
The  solution  is  then  electrolyzed  with  a  current  of  NDioo  =  0.3 
to  0.4  ampere,  which  is  strengthened  to  about  1  ampere  after  the 
greater  part  of  the  metal  has  been  removed. 

*  Ber.,  12,  759  (1879).  f  Z.  anal.  Chem.,  37,  284  (1898). 


CADMIUM 


177 


According  to  the  quantity  of  cadmium  and  the  strength  of 
current  used,  the  electrolysis  may  require  12  hours  or  longer. 
Heating  the  solution  to  50°  or  60°  hastens  the  process. 

At  the  last,  the  solution  is  tested  for  cadmium  by  heating  under 
the  hood  a  little  of  the  liquid  with  dilute  sulphuric  acid  until  all 
the  hydrogen  cyanide  has  been  expelled  and  then  adding  an  excess 
of  hydrogen-sulphide  water.  If  no  yellow  precipitate  or  yellow 
coloration  is  noticeable,  the  solution  is  free  from  cadmium  *  and  the 
deposit  should  be  washed  with  water  before  breaking  the  circuit. 
It  is  finally  washed  with  alcohol  and  dried  as  described  under 
Copper. 

The  following  table  gives  the  conditions  which  have  given  good 
results  in  the 

RAPID    DEPOSITION    OF    CADMIUM    FROM    ALKALI-CYANIDE 

SOLUTION.! 


Experiments  performed  by 

Exner. 

Davison. 

Flora. 

Electrode    . 

Dish  and 
rotating 
spiral 

2  gms.  KCN 
and  5  gms. 
NaOH 

120  cc. 

0.55  gm.  as 
sulphate 

Hot 

8  volts 
600 
10  to  15 

Dish  and 
rotating 
spiral 

4  gms.  KCN 
and  2  gms. 
NaOH 

90  to  125  cc. 

0.35  gm.  as 
nitrate 

Hot 

4  volts 
600 
15 

Dish  and 
rotating 
spiral 

5  gms.  KCN 
and  1  gm. 
NaOH 

90  to  125  cc. 

0.35  to  0.45 
gm.  as 
nitrate 

Hot 

5.5  volts 
600 
20 

Rotating  cru- 
cible as  ca- 
thode 

0.5  to  1.5  gms. 
KCN  and  0.5 
to  1.5  gms. 
NaOH 

65  to  70  cc. 

0.1  to  0.15  as 
sulphate  or 
nitrate 

Room  temper- 
ature 

7.6  to  8  volts 
600 
30  to  45 

Electrolyte  con- 
tained. 

Volume  

Quantity  of  metal.  . 
Temperature  .... 

Potential  

Revolutions  

Time  in  minutes  

*  It  should  be  remembered  that  the  test  will  not  be  obtained  if  the  solu- 
tion is  too  acid. 

t  General  references  to  the  literature  on  the  rapid  electrodeposition  of 
cadmium  in  various  solutions:  Medway,  Am.  J.  Science  [41,  18,  56  (1904);  Z. 
anorg.  Chem.,  42,  114  (1904).  A.  Fischer  and  Boddaert/Z.  Elektrochem,  10, 
945  (1904).  Flora,  Z.  anorg.  Chem.,  47, 1, 13,  20  (1905).  Exner,  J.  Am.  Chem 
Soc.,  25,  896  (1903).  Davison,  Thesis,  U.  of  Pa.,  1906.  H.  J.  S.  Sand,  J. 
Chem.  Soc.,  London,  91,  373  (1907).  Smith  and  Kollock,  J.  Am.  Chem.  Soc., 
27,  1527  (1905). 


178  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Deposition  of  Cadmium  from  Oxalate  Solution.* 

The  metal  may  be  obtained  in  compact  form,  with  a  bright 
metallic  luster,  by  the  electrolysis  of  a  hot  solution  of  the  complex 
ammonium  cadmium  oxalate  in  the  presence  of  free  oxalic  acid. 
The  cadmium  salt  is  dissolved  in  a  weighed  platinum  dish  by 
heating  it  with  20  to  25  cc.  of  water.  When  all  the  salt  has  dis- 
solved, a  hot,  filtered  solution  of  10  gms.  ammonium  oxalate  in 
80  to  100  cc.  of  water  is  added  and  the  electrolysis  is  carried  out 
with  a  current  of  NDioo  =  0.5  to  1  ampere  at  a  potential  of  3  to 
3.4  volts.  During  the  entire  time  the  solution  is  kept  at  a  temper- 
ature of  70°  to  75°.  As  soon  as  the  action  of  the  current  has  begun, 
a  few  cubic  centimeters  of  an  oxalic-acid  solution,  saturated  at 
the  room  temperature,  are  poured  upon  the  watch  glass  which 
covers  the  dish,  and  the  electrolyte  is  kept  slightly  acid  by  the 
addition  from 'time  to  time  of  a  little  of  this  solution.  The  end 
of  the  reaction  is  determined  by  removing  a  little  of  the  solution 
with  a  piece  of  glass  tubing,  acidifying  it  with  hydrochloric  acid 
and  adding  hydrogen  sulphide.  The  metal  must  be  washed  with 
water  without  interrupting  the  current.  The  largest  quantity 
of  metal  which  can  be  deposited  satisfactorily  in  this  way  is  0.15 
to  0.16  gm.  and  the  time  required  is  about  three  hours. 

The  method  has  proved  satisfactory  in  analytical  practice.  If 
the  cadmium  is  present  as  sulphide,  about  0.5  gm.  of  it  is  dissolved 
in  hydrochloric  acid,  the  solution  is  evaporated  to  dryness  and  the 
residual  cadmium  chloride  is  treated  as  described.  Instead  of 
using  a  platinum  dish  as  cathode,  good  results  can  be  obtained  and 
larger  quantities  of  metal  deposited  by  using  a  brass  gauze  cathode, 
of  the  shape  recommended  by  Winkler  (p.  59),  which  has  been 
amalgamated.  Paweck,f  who  first  used  this  kind  of  an  electrode, 
amalgamated  it  by  first  dipping  the  gauze  in  nitric  acid  to  clean 
it,  washing  it  with  water,  and  then  using  it  as  cathode  for  45  min- 
utes to  an  hour  with  a  current  of  0.1  to  0.2  ampere  in  a  solution 
containing  about  0.6  gm.  of  mercuric  chloride,  5  cc.  of  concentrated 
nitric  acid  and  200  cc.  of  water;  a  platinum  wire  was  used  as  anode. 
The  quantity  of  mercury  deposited  in  this  way  upon  the  brass 
gauze  must  not,  of  course,  be  so  large  that  it  will  drop  off  in 
handling  the  electrode.  The  amalgamated  electrode  is  raised 

*  A.  Classen  and  v.  Reis,  Ber.,  14,  1628  (1881);  A.  Classen,  ibid.,  17,  2060 
(1884). 

t  Z.  Elektrochem.,  6,  221  (1898). 


INDIUM  179 

quickly  from  the  bath  and  washed  successively  with  dilute  hydro- 
chloric acid,  water  and  absolute  alcohol.  It  must  be  dried  very 
carefully,  by  placing  it  at  some  distance  above  a  hot  asbestos 
plate,  and  then  kept  in  a  desiccator  until  it  is  weighed. 

After  the  cadmium  has  been  deposited  upon  such  an  amal- 
gamated electrode,  it  must  be  washed  and  dried  as  before.  After 
it  has  been  weighed,  the  electrode  is  placed  in  quite  concentrated 
hydrochloric  acid,  and,  when  the  evolution  of  gas  has  stopped, 
washed  and  dried  as  above;  it  is  then  ready  for  use  again.  Thus 
a  large  number  of  analyses  may  be  carried  out  with  the  same 
electrode  after  it  has  been  once  amalgamated. 

Besides  the  above  three  distinctly  different  ways  for  determining 
cadmium  electrolytically,  quite  a  number  of  other  methods  have 
been  proposed  some  of  which  do  not  give  satisfactory  results,  and 
the  others  possess  no  advantages  over  those  that  have  been  given. 
Some  of  these  methods  are  mentioned  in  the  footnote.* 

Indium. 
At.  Wt.  =  114.8.     Elec.  Equiv.  =  0.397  mg.  for  In++  +  ions. 

Rapid  Deposition  from  Formic-acid  Solution. 

L.  M.  Dennis  and  W.  C.  Geer  f  have  obtained  excellent  deposits 
of  indium  upon  platinum  and  found  the  determination  sufficiently 
accurate  to  use  for  determining  the  atomic  weight  of  this 
element. 

Yellow  indium  oxide  is  dissolved  on  the  water  bath  in  six  times 
its  weight  of  normal  sulphuric  acid,  carefully  avoiding  an  excess 
of  acid.  To  the  resulting  solution,  25  cc.  of  formic  acid  (sp.  gr. 
1.2)  are  added,  then  5  cc.  of  ammonia  (sp.  gr.  0.908)  and  the  solu- 
tion is  diluted  to  200  cc.  The  quantity  of  metal  in  the  elec- 
trolyte may  vary  within  wide  limits  (0.2  to  1.5  gms.)  and  likewise 
the  current  density  (NDioo  =  9  to  12  amperes).  The  results  are 
equally  good  whether  the  indium  is  deposited  upon  a  rotating 
crucible,  upon  a  platinum  dish,  or  upon  a  stationary  platinum- 

*  D.  L.  Wallace  and  E.  F.  Smith  use  an  acetate  solution  and  also  a  phos- 
phate solution,  Z.  Elektrochem.,  4,  259  (1898);  5,  167  (1898).  Balachowsky 
adds  acetic  acid,  urea,  formaldehyde  and  acetaldehyde  to  the  bath,  Z.  Elek- 
trochem., 7,  272  (1900).  See  also  Separation  of  Cadmium  from  Silver. 

f  Ber.,  37,  961  (1904). 


180  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

foil  electrode.  Apparently  air  has  no  effect  upon  the  deposit. 
A.  Thiel  *  found  that  after  the  indium  was  dissolved  off  the 
electrode,  the  platinum  was  attacked,  as  in  the  determination  of 
zinc;  but  Dennis  and  Geer  noticed  the  formation  of  platinum 
black  only  when  too  little  formic  acid  was  used  in  the  electrolysis 
of  indium  chloride. 

*Ber.,  37,  176(1904). 


IRON  181 


GROUP  III. 

The  metals  of  this  group,  iron,  nickel,  and  cobalt,  are  above 
hydrogen  in  the  voltage  series  and  do  not  show  enough  over  voltage 
toward  the  evolution  of  hydrogen  to  permit  their  deposition  from 
acid  solution  except  in  the  form  of  amalgams  or  from  solutions 
of  weak  organic  acids  which  are  susceptible  to  anodic  oxidation. 
While  the  acid  is  oxidized  at  the  anode,  the  acidity  is  decreased 
at  the  cathode  if  hydrogen  ions  are  discharged. 

Iron. 

At.  Wt.  =  55.84.  Elec.  Equiv.  =  0.29  mg.  for  Fe  +  +  ions. 
Elec.  Potential  =  +  0.340  volt.  Overvoltage  of  H2  =  0.08 
volt  from  an  alkaline  solution. 

Of  the  many  methods  proposed  for  the  electrolytic  determination 
of  iron,  the  best  is  that  of  the  deposition  of  the  metal  from  a  solu- 
tion of  the  complex  ammonium  f  erro-oxalate  or  of  ammonium  f  erri- 
oxalate.*  Some  authors  have  raised  the  objection  that  the  metal 
deposit  will  contain  carbon  but  the  daily  experience  of  the  author 
during  the  last  25  years  has  proved  that,  if  the  work  is  properly 
carried  out,  the  iron  deposit  is  entirely  free  from  carbon  and  the 
results  are  quantitatively  correct.  S.  Avery  and  Benton  Dalesf 
were  the  first  to  assert  that  an  iron  deposit  obtained  by  the  ammo- 
nium-oxalate  method  was  contaminated  with  carbon.  As  a  result 
of  this  assertion,  H.  Verwer  and  F.  Groll,}  in  the  author's  labora- 
tory subjected  the  method  to  the  closest  scrutiny  and  the  results 
of  their  studies  showed  that  the  use  of  ammonium  oxalate  in  the 
electrolysis  of  iron  salts  does  not  cause  a  simultaneous  deposition 
of  carbon,  either  as  such  or  as  carbide,  and  that  the  iron  is  deposited 
quantitatively. 

By  further  investigation,  however,  Verwer  §  found  that  the 
deposit  may  contain  carbon  in  case  the  electrolysis  is  prolonged 

*  A.  Classen  and  v.  Reis.,  Ber.,  14,  1622  (1881). 

t  Ibid.,  32,  64  (1899). 

J  Ibid.,  32,  806  (1899). 

§  Chem.-Ztg.,  25,  792  (1901). 


182  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

beyond  the  time  necessary  for  the  complete  removal  of  the  iron. 
The  author's  directions,  if  strictly  followed,  will  not  give  rise  to 
this  error.  Verwer's  experiments  showed  that  the  deposition  of 
carbon  was  not  due  to  the  use  of  too  high  a  voltage  but  rather 
to  the  formation  of  carbonate  at  the  anode  (cf.  p.  51)  and  the 
subsequent  reduction  of  CO 3  at  the  cathode.  In  this  way  free 
carbon  is  deposited  upon  the  iron.  This  can  be  shown  by  the 
following  experiment.  If  a  number  of  deposits  of  iron  are  made 
and  the  current  stopped  each  time  before  all  the  iron  is  removed 
from  the  solution,  careful  examination  of  the  deposits  will  fail  to 
reveal  the  presence  of  any  carbon.  If  now  an  electrode  with  an 
iron  deposit  upon  it  is  used  as  cathode  in  an  ammonium-carbonate 
solution,  or  in  an  ammonium-oxalate  solution  from  which  all  the 
iron  has  been  removed  by  12  hours  electrolyzing,  then  the  deposit 
will  become  strongly  contaminated  with  carbon. 

Other  experimenters  have  also  obtained  good  results  4  with  the 
method.  Thus  Schudl,*  in  using  electrolytic  iron  for  the  stand- 
ardization of  a  potassium-permanganate  solution,  found  that  the 
results  obtained  compared  favorably  with  values  obtained  by  other 
methods.  A.  Neuburger  f  obtained  similar  results. 

If  to  the  oxalate  solution  such  substances  as  tartaric  acid  or 
citric  acid  (E.  F.  Smith)  are  added,  then  the  iron  deposit  is  more 
likely  to  contain  carbon. 

The  solution  in  which  the  iron  is  to  be  determined  may  contain 
a  ferrous  or  ferric  salt.  If  a  solution  of  a  ferrous  salt  is  treated 
with  ammonium  oxalate,  there  is  produced  a  deep,  yellowish-red 
precipitate  of  ferrous  oxalate,  soluble  in  an  excess  of  the  reagent 
to  a  yellowish-red  solution  of  ammonium  ferro-oxalate. 

Ferric  salts  are  not  precipitated  but  if  the  yellow  solution  of 
ferric  chloride  is  treated  with  sufficient  ammonium  oxalate,  a 
solution  of  ammonium  ferri-oxalate  results  which  has  a  light  green 
color.  It  is  interesting  to  note  that  the  complex  ferrous  oxalate 
forms  a  yellow  solution  and  the  complex  ferric  oxalate  a  green  one, 
whereas  the  color  of  ferrous  chloride  is  green  and  that  of  ferric 
chloride  is  yellow. 

If  the  solution  of  ammonium  ferri-oxalate  is  subjected  to  elec- 
trolysis, first  of  all  the  corresponding  ferrous  salt  is  formed  by 
reduction  and  the  latter  eventually  gives  a  deposit  of  iron  on  the 

*  Cf.  Treadwell-Hall,  "Quantitative  Analysis." 
t  Z.  Elektrochem.,  11,  77  (1904). 


IRON  183 

cathode;  thus  the  solution  changes  from  yellow  into  red  and 
finally  becomes  colorless.  It  is  evident,  too,  that  the  electrolysis 
will  take  place  more  quickly  on  starting  with  the  ferrous  salt 
because  otherwise  some  of  the  current  is  used  up  in  accomplish- 
ing the  preliminary  reduction  (cf.  the  rapid  determination  of  iron, 
below). 

The  electrolysis  of  the  ammonium  double  oxalates  takes  place 
smoothly  without  any  intermediate  formation  of  insoluble  iron 
compounds.  Only  when  nitrates  are  present  are  flocks  of  ferric 
hydroxide  sometimes  observed.  Nitrates,  therefore,  must  be 
removed  before  the  electrolysis  by  evaporating  with  sulphuric 
acid.  The  greater  part  of  the  excess  acid  is  expelled,  and  then, 
after  diluting  and  boiling  until  all  the  sulphate  has  dissolved,  the 
last  traces  of  the  free  acid  are  removed  by  neutralization  with 
ammonia.  The  ammonium  sulphate  thus  formed  has  a  favorable 
effect  upon  the  analysis  and  increases  the  conductivity  of  the 
solution. 

If  the  solution  contains  free  hydrochloric  acid  it  is  best  to  remove 
it  by  evaporating  on  the  water  bath.  Chlorides,  however,  do  not 
exert  a  harmful  effect. 

Procedure.  If  the  solution  to  be  electrolyzed  does  not  contain 
more  than  1  gm.  of  iron,*  from  6  to  8  gms.  of  ammonium  oxalate 
are  dissolved  by  heating  with  as  little  water  as  possible  in  a  plat- 
inum dish,  and,  while  constantly  stirring,  the  iron  solution  is 
gradually  added,  f  The  solution  is  then  diluted  with  water  to  a 
volume  of  100  to  150  cc.  and  the  platinum  disk,  which  is  to  serve 
as  anode,  is  immersed  in  the  solution  until  it  is  just  covered  by 
the  liquid.  The  temperature  of  the  electrolyte  does  not  matter 
much  in  this  case  and  the  analysis  may  be  carried  out  at  the  labo- 
ratory temperature  or  at  65°.  In  the  former  case  the  current 

*  Ferrous  ammonium  sulphate,  FeSO^CNH^SO^G  H2O,  may  be  used  for 
practice.  The  salt  is  easily  obtained  pure  and  contains  exactly  one-seventh 
iron.  The  salt  should  be  dissolved  in  water  containing  a  few  drops  of  sul- 
phuric acid. 

t  It  is  not  advisable  to  add  the  ammonium  oxalate  to  the  ferrous  solution 
because  ferrous  oxalate  will  be  precipitated  and  can  be  dissolved  only  by  long 
heating  with  an  excess  of  oxalate  solution.  If  only  ferric  salts  are  present, 
there  is  no  need  of  taking  the  above  precaution  as  no  precipitate  forms. 

Potassium  oxalate  cannot  be  used  instead  of  ammonium  oxalate,  as  it 
becomes  partly  changed  to  potassium  carbonate  during  the  electrolysis  and 
forms  a  precipitate  of  basic  ferrous  carbonate. 


184 


QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 


density  should  be  NDi0o  =  l  to  1.5  amperes  and  in  the  latter  case, 
0.5  to  1  ampere.  The  corresponding  potential  difference  between 
the  electrodes  is  from  4  to  2  volts. 

After  the  solution  has  become  perfectly  colorless,  a  little  of  it 
is  heated  with  a  drop  of  nitric  acid  (to  oxidize  any  iron  to  the 
ferric  condition),  an  excess  of  hydrochloric  acid  is  added  and  a 
liberal  quantity  of  potassium  thiocyanate  solution.*  If  no  red 
coloration  is  obtained  by  this  test,  the  current  is  turned  off,  the 
solution  at  once  poured  out  and  the  dish  is  washed  several  times 
with  cold  water  and  finally  with  alcohol.  It  is  dried  at  70°  to  80° 
and  weighed  after  cooling  in  a  desiccator. 

The  iron  deposit  has  a  lustrous,  steel-gray  color,  adheres  firmly 
to  the  dish,  either  on  a  polished  or  roughened  surface,  and  can  be 
kept  all  day  in  the  air  without  oxidation. 

The  conditions  for  the  rapid  electrolysis  of  an  oxalate  solution 
are  given  in  the  following  table. 

RAPID    DEPOSITION    OF   IRON   FROM    OXALATE    SOLUTION,  t 


Experiments  performed  by 

A.  Fischer  at  Aachen. 

Exner. 

Electrode  

Dish  and  rotating  disk 

7  to  7.5  gms.  of  ammonh 
saturated  solution 

120  cc. 
0.2  gm.  as  Mohr's  salt 
85° 
6  to  7  volts 
7  amp. 
600 
30  min: 

Dish  and  rotating  spiral 

im  oxalate  and  1  cc.  of  a 
of  oxalic  acid 

125  cc. 
0.25  gm.  as  ferric  alum 
Hot 
7.4  to  7.5  volts 
7  amp. 
800 
25  to  35  min. 

Electrolyte  contained  .  . 
Volume             

Quantity  of  metal  

Temperature  

Potential  

Current  strength  .  .    . 

Revolutions    

Time        

*  It  is  to  be  remembered  that  oxalates  tend  to  prevent  the  above  test  for 
iron.  The  original  oxalate  content  is,  however,  considerably  reduced  by  the 
action  of  the  current  and  the  test  will  be  obtained  with  traces  of  iron  if  an 
excess  of  hydrochloric  acid  and  considerable  potassium  thiocyanate  are  used. 

t  General  references  to  the  literature  on  the  rapid  electrolytic  determina- 
tion of  iron:  Exner,  J.  Am.  Chem.  Soc.,  25,  896  (1903).  A.  Fischer,  Chem.- 
Ztg.,  31,  25  (1907).  E.  F.  Smith  and  Kollock,  J.  Am.  Chem.,  Soc.,  27, 1255, 
1527  (1905).  Frary,  Z.  Elektrochem.,  13,  308  (1907);  Z.  angew.  Chem.,  20, 
1897  (1907). 


NICKEL  185 

According  to  the  time  required  in  the  above  experiments  it 
would  appear  that  it  makes  no  difference  whether  the  iron  is 
originally  present  as  ferrous  or  as  ferric  salt,  whereas  with  sta- 
tionary electrolytes  more  time  is  required  with  the  latter.  The 
iron  deposits  obtained  by  the  rapid  method  are  perfectly  free  from 
carbon  if  the  current  is  not  continued  too  long  after  all  the  iron  is 
deposited.  A.  Fischer  begins  adding  a  little  oxalic  acid  after 
the  analysis  has  been  in  progress  about  15  minutes  and  then  adds 
0.2  cc.  every  3  minutes. 

For  rapid  analysis  by  the  aid  of  magnetic  stirring,  see  page  78. 

Nickel. 

At.  Wt.  =  58.68.  Elec.  Equiv.  =  0.304  mg.  for  Ni++  ions. 
Elec.  Potential  =  +  0.228  volt.  Overvoltage  of  H2  =  0.03- 
0.21  volt. 

Numerous  methods  have  been  proposed  for  the  electrolytic 
determination  of  nickel,  but  here  only  the  deposition  from  am- 
moniacal  and  from  oxalate  solutions  will  be  discussed. 

Deposition  of  Nickel  from  Ammoniacal  Solution. 

This  method  was  first  proposed  and  tested  by  W.  Gibbs  in  1864. 
H.  Fresenius  and  Bergmann  *  determined  the  best  conditions  for 
carrying  out  the  electrolysis  and  since  that  time  the  electrolytic 
determination  of  nickel  has  been  regarded  as  one  of  the  most  satis- 
factory methods  for  determining  this  element.  It  is  important 
that  a  large  excess  of  ammonia  be  present;  this  condition,  which 
has  been  recognized  in  practice  for  a  long  time,  has  been  explained 
by  Foerster  f  on  the  basis  of  the  theory  of  electrolytic  dissocia- 
tion. In  the  ammoniacal  solution  of  nickel  sulphate,  for  exam- 
ple, the  metal  is  present  as  complex  nickel-ammonium  sulphate, 
[Ni(NH3)4]S04,  and  this  salt  is  dissociated  primarily  into  SO^ 
and  the  complex  nickel-ammonia  cation,  [Ni(NH3)4]++.  Like  all 
complex  ions,  the  latter  itself  undergoes  a  slight  dissociation  and 
there  is,  therefore,  the  following  state  of  equilibrium  in  the  solution 
Ni++  +  4  NH3  <=±  [Ni(NHs)  4]++. 

*  Z.  anal.  Chem.,  19,  320  (1880). 

t  Z.  angew.  Chem.,  19,  1884,  1889  (1906). 


186  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Inasmuch  as  hydroxyl  ions  are  present  in  an  ammoniacal  solu- 
tion, there  is  a  tendency  for  nickel  hydroxide  to  precipitate  in  case 
the  concentration  of  the  Ni  ions  becomes  sufficiently  great  at  any 
time.  The  concentration  of  Ni  ions,  however,  can  be  kept  as  low 
as  one  pleases  by  making  the  ammonia  content  high.  Under 
these  conditions  the  nickel  ions  are  discharged  at  the  cathode  in  a 
metallic  condition,  whereas,  if  insufficient  ammonia  is  present,  a 
dark-colored  deposit  containing  oxide  is  obtained. 

There  is  often  a  tendency  for  the  deposit  to  scale  off  the  cathode. 
This  is  avoided  by  the  use  of  a  platinum  gauze  cathode. 

If  a  solution  of  nickel  sulphate  containing  free  sulphuric  acid 
is  to  be  analyzed,  it  is  neutralized  with  ammonia,  sp.  gr.  0.9,  and 
24  cc.  are  added  in  excess;  the  clear,  blue  solution  thus  obtained 
is  diluted  to  100  cc.  In  the  liquid,  which  should  preferably  con- 
tain not  more  than  0.1  gm.  of  nickel,  from  4  to  5  g.  of  ammonium 
sulphate  are  dissolved  to  increase  the  conductivity,  and  the  elec- 
trolysis is  conducted  with  a  current  of  0.7  to  1.0  ampera  with  a 
gauze  cathode.  The  deposition  of  the  nickel  is  then  accomplished 
in  about  45  minutes.  The  solution  is  proved  to  be  free  from  nickel 
by  testing  a  portion  of  it  with  ammonium  sulphide  or  with  potas- 
sium thiocarbonate;  with  the  former  reagent  a  brown  coloration 
is  obtained  and  with  the  latter  a  pink  one.  The  current  is  turned 
off  and  the  electrode  washed  and  dried  in  the  usual  manner.  The 
deposit  is  bright  and  lustrous,  hardly  to  be  distinguished  from  the 
platinum. 

Until  recently  the  electrolysis  of  a  nitrate  solution  has  led  to 
bad  results;  it  was  usual,  therefore,  to  evaporate  nitrate  solutions 
with  sulphuric  acid  before  adding  water  and  ammonia.  A.  Thiel  * 
has  shown,  however,  that  it  is  possible  to  electrolyze  a  solution 
containing  nitrate  if  certain  precautions  are  taken.  This  fact  is 
of  importance  in  those  cases  where  the  separation  of  copper  and 
nickel  in  a  nitric-acid  solution  is  involved  as  it  enables  one  to  avoid 
the  time-consuming  operation  of  evaporating  with  sulphuric  acid. 

Thiel  first  convinced  himself  that  the  poor  deposits  obtained 
with  the  use  of  solutions  containing  nitrate  were  due,  as  A.  Windel- 
schmidt  f  had  previously  recognized,  less  to  the  presence  of 
nitrate  than  to  the  presence  of  nitrite.  The  nitrous  acid  is 
formed  either  in  the  solution  of  an  alloy  in  nitric  acid  or  during  the 
progress  of  the  electrolysis.  After  it  was  found  that  simply  boil- 
*  Z.  Elektrochem.,  14,  201  (1908).  f  Dissertation,  Miinster,  1907. 


NICKEL  187 

ing  the  acid  solution  sufficed  to  free  it  from  nitrous  acid,  other 
difficulties  were  encountered  due  to  the  fact  that  under  certain 
conditions  the  platinum  anode  is  attacked  by  the  ammoniacal 
solution.  In  such  cases  the  weight  of  the  nickel  is  found  to  be  too 
high.  Thiel  succeeded  in  overcoming  this  difficulty  by  employing, 
instead  of  the  spiral  platinum  anode,  a  straight  wire  of  passive 
iron.  An  iron  wire  of  1.5  mm.  diameter  is  cleaned  with  hydro- 
chloric acid  and  made  passive  by  dipping  it  for  a  short  time  in 
nitric  acid  (sp.  gr.  1.4)  and  immediately  washing  it  with  water 
and  with  alcohol.  The  dull  surface  of  the  metal  is  unchanged  for 
a  long  time  on  exposure  to  the  atmosphere.  A  straight  wire  is 
better  in  this  case  than  a  spiral,  because  the  ferric  hydroxide, 
with  which  the  wire  gradually  becomes  coated  during  the  electroly- 
sis, is  more  likely  to  scale  off  from  a  spiral  and  this  might  lead  to 
contamination  of  the  deposit. 

The  analysis  is  carried  out  as  follows:  The  nitric-acid  solution 
of  nickel  is  boiled  to  expel  traces  of  nitrous  acid  and,  after  cooling, 
it  is  neutralized  with  ammonia.  Enough  more  of  the  reagent  is 
added  so  that  in  200  cc.  of  electrolyte  about  80  cc.  of  ammonia 
(sp.  gr.  0.91)  will  be  present.  If  insufficient  ammonia  is  present, 
some  oxide  is  deposited  with  the  nickel.  As  cathode  the  cylinder 
of  platinum  gauze  (Fig.  24,  p.  60)  is  used  and  as  anode  the  passive 
iron  wire.  The  current  density  is  NDioo  =  5  amperes  and  the 
potential  difference  between  the  electrodes  is  14  volts  at  the  start. 
It  is  not  necessary  to  heat  the  beaker  with  a  flame  and,  in  fact, 
this  is  not  advisable  on  account  of  the  large  losses  of  ammonia 
that  will  then  result  by  evaporation.  Owing  to  the  high  current 
density  used,  the  electrolyte  becomes  heated  to  about  70°  and  the 
potential  of  the  bath  sinks  to  about  10  volts. 

The  electrolysis  is  continued  until  the  solution  is  absolutely 
colorless  and  there  is  a  strong  liberation  of  gas  at  the  cathode; 
this  takes  place  with  0.1  to  0.3  gm.  of  nickel  at  the  end  of  45  or 
50  minutes.  Then,  without  breaking  the  circuit,  the  cathode  is 
gradually  raised  from  the  solution  while  washing  it  with  a  stream 
of  water  from  a  wash  bottle.  The  electrode  is  dried  in  the  usual 
way  before  weighing  it.  The  anode  is  replaced  by  a  fresh  piece  of 
passive  iron  wire,  the  cathode  is  again  put  in  the  solution  and  the 
electrolysis  is  continued  for  5  minutes  more.  The  cathode  is 
washed,  dried  and  weighed  and  if  this  weight  is  not  the  same  as 
before  the  process  is  repeated  again.  Thiel  found  that  the  second 


188  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

weighing  was  usually  a  few  tenths  of  a  milligram  lower  than  the 
first  but  that  the  third  weighing  was  usually  the  same  as  the 
second.  The  last  weight  obtained  was  regarded  as  correct. 

The  deposited  nickel  is  dull  gray;  duplicate  analyses  agree 
well. 

This  method  of  continuing  the  electrolysis  until  two  consecutive 
weighings  agree  is  recommended  by  Thiel  because  the  test  for 
nickel  with  ammonium  sulphide  is  not  reliable  in  a  strongly 
ammoniacal  solution. 

The  very  sensitive  reaction  for  nickel  that  depends  upon  the 
formation  of  nickelic  hydroxide  by  the  action  of  bromine  and 
caustic  alkali  gives  reliable  results  only  in  the  absence  of  ammo- 
nium salts.  A  few  cubic  centimeters  of  the  solution,  therefore, 
may  be  evaporated  to  dryness  on  a  large  porcelain  crucible  cover, 
the  ammonium  salts  expelled  by  gentle  ignition  and  the  residue 
barely  moistened  with  aqua  regia  and  dissolved  in  water.  This 
solution  is  treated  with  caustic  potash  solution  and  bromine  vapors 
are  brought  in  contact  with  it.  As  little  as  0.00004  gm.  of  nickel 
will  at  once  give  a  black  precipitate;  one  tenth  as  much  nickel 
will  be  shown  when  the  solution  turns  yellow  owing  to  the  absorp- 
tion of  bromine. 

Chloride  solutions  also  were  regarded  as  unsuitable  for  the 
electrolytic  determination  of  nickel  in  ammoniacal  solution  until 
F.  Oettel  *  showed  that  satisfactory  deposits  could  be  obtained 
from  them.  Ammonium  chloride  is  added  to  the  electrolyte  to 
increase  its  conductivity,  about  10  gms.  being  used  for  1  gm.  of 
nickel;  a  larger  quantity  of  the  salt  does  no  harm.  The  quantity 
of  free  ammonia  present,  sp.  gr.  0.92,  must  reach  at  least  20  per 
cent  by  volume,  as  otherwise  a  black  deposit  of  nickel  oxide  will 
be  obtained  upon  the  anode  and  some  oxide  will  be  present  in  the 
nickel  on  the  cathode.  With  a  current  density  of  NDioo  =  0.4 
ampere,  1  gm.  of  nickel  is  usually  deposited  in  7  or  8  hours  and  as 
cathode  the  gauze  electrode  has  proved  most  satisfactory.  The 
end  of  the  electrolysis  is  determined  by  one  of  the  several  tests 
that  have  been  given. 

A  number  of  investigators  have  studied  the  rapid  electrodeposi- 
tion  of  nickel  and  some  of  the  results  obtained  in  ammoniacal  solu- 
tions are  given  in  the  table  on  page  189. 

Concerning  the  deposition  with  magnetic  stirring,  see  page  78. 
*  Z.  Elektrochem.,  1,  194  (1894). 


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190          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Deposition  of  Nickel  from  Oxalate  Solution. 

Nickel,  when  deposited  from  a  solution  of  the  double  oxalate, 
is  always  contaminated  with  carbon.  Foerster  regarded  the 
formation  of  carbon  compounds  on  the  cathode  as  due,  in  this 
case,  to  the  migration  of  colloidal  substances  to  the  cathode  and 
supported  this  assumption  by  suggesting  the  possibility  of  oxalic 
acid  being  transformed  into  glycollic  acid  *  with  the  accompany- 
ing formation  of  resin-like  substances.  The  investigations  of  A. 
Mailhe,f  Henseling  J  and  others  indicate  that  the  carbon  deposi- 
tion is  more  in  the  nature  of  a  purely  chemical  reaction,  in  which 
the  nickel  probably  acts  as  a  catalyzer.  To  explain  the  phe- 
nomenon a  number  of  experiments  have  been  performed  in  the 
author's  laboratory. 

In  most  cases  the  analyst  is  not  so  much  concerned  in  the 
determination  of  nickel  in  a  solution  of  a  pure. nickel  salt  as  in  the 
determination  and  separation  of  nickel  from  other  elements.  A 
separation  of  nickel  from  chromium,  aluminium  and  manganese 
is  not  accomplished  electrolytically  by  the  above-described  am- 
monia method  but  is  effected  by  the  electrolysis  of  an  oxalate 
solution.  In  such  cases,  other  metals,  such  as  copper,  are  deposited 
with  the  nickel  so  that  it  is  necessary  to  redissolve  the  precipitate 
and  free  it  from  impurities  including  carbon.  In  the  purified 
solution,  the  nickel  may  be  determined  by  the  ammonia  method. 
For  reasons  such  as  this,  the  oxalate  method  of  electrolyzing 
nickel  solutions  cannot  well  be  discarded. 

For  carrying  out  the  method  the  electrolyte  is  prepared  in 
much  the  same  way  as  in  the  determination  of  iron.  About 
25  cc.  of  the  nickel  solution  (chloride  or  sulphate,  but  not  the 
nitrate)  are  treated  with  4  to  5  gms.  of  ammonium  oxalate, 
which  is  dissolved  by  heating  the  liquid,  and,  after  diluting  to 
100  to  120  cc.,  the  electrolysis  is  carried  out  with  a  current  of 
NDioo  =  1  ampere.  The  deposition  of  0.3  gm.  nickel  requires 
about  3  hours.  The  end  of  the  reaction  is  determined  and  the 
•treatment  of  the  deposit  before  weighing  is  carried  out  as  described 
on  page  186. 

*  Avery  and  Dales,  Ber.,  32,  2237  (1899)  obtained  glycollic  acid  by  the 
electrolysis  of  a  nearly  boiling  oxalic-acid  solution  with  a  current  of  10  amperes 
and  10  volts  for  2.5  hours.  These  conditions  are  altogether  different  from 
those  which  prevail  in  the  metal  determination. 

t  Chem.-Ztg.,  31,  1083  (1907).  J  Dissertation,  Karlsruhe,  1906. 


COBALT 


191 


The  following  table  gives  some  of  the  results  obtained  with 
stirred  electrolytes. 
RAPID  DEPOSITION  OF  NICKEL  FROM  OXALATE  SOLUTION. 


Experiments  performed  by  A.  Fischer  and  Boddaert  in 
the  laboratory  at  Aachen. 

Electrode  

Dish  and  rotating  disk 

15  gms.  ammonium  ox- 
alate 

125  cc. 
0.2  to  0.33  gm. 
60°  to  95° 
6  to  5  volts 
7.5  to  8  amp. 
600  to  800 
50  min. 

Dish  and  f-otating  disk 

80  cc.  of  a  cold,  satu- 
rated solution  of  ammo- 
nium oxalate 

125  cc. 
0.2  to  0.33  gm. 
22°  to  60° 
7  to  6.2  volts 
8  amp. 
600  to  800 
40  min. 

Electrolyte  contained  .  . 
Volume  

Quantity  of  metal 

Temperature  

Potential  of  the  bath... 
Current  strength  
Revolutions  

Time  

With  regard  to  the  carbon  content  of  the  deposits  obtained  by 
the  rapid  method,  what  was  said  on  page  190  holds  true  here  also. 


Cobalt. 

At.  Wt.  =  58.97.     Elec.    Equiv.  =  0.306    mg. 
Elec.  Potential  =  +  0.232  volt. 


for 


ons. 


The  methods  given  for  the  determination  of  nickel  apply 
without  any  change  for  the  determination  of  cobalt.  In  the 
oxalate  method,  the  effect  of  this  element  is  not  so  marked  but 
there  is  still  some  deposition  of  carbon  upon  the  cathode. 

It  is  quite  common  to  determine  nickel  and  cobalt  together  by 
electrolysis  and  determine  the  nickel,  however,  independently 
as  the  salt  of  dimethyl  glyoxime. 

The  electrolytic  determination  of  cobalt,  however,  is  likely 
to  give  results  which  are  somewhat  too  high,  probably  on  account 
of  oxidation  of  the  deposit,  or  too  low,  owing  to  formation  of  a 
little  peroxide  at  the  anode.  Often  the  precipitated  cobalt  has 
a  brown  or  black  appearance.  Bright  deposits  can  be  obtained 
by  adding  sodium  hypophosphite  to  the  bath,  but  the  results 
are  then  too  high  because  of  the  deposition  of  cobalt  phosphide. 


192          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Perkin  *  states  that  the  most  satisfactory  results  are  obtained 
when  the  electrolyte  contains  primary  sodium  phosphate  and 
phosphoric  acid. 

Procedure. — To  the  neutral  solution  of  the  cobalt  salt  con- 
taining about  0.5  gm.  of  the  metal  in  80  cc.  of  water,  add  2  cc. 
of  a  5  per  cent  solution  of  phosphoric  acid  and  20-25  cc.  of  a 
10  per  cent  solution  of  primary  sodium  phosphate  (NaH^PO/O  in 
water.  Stir  well  while  adding  the  sodium  phosphate  solution  from 
a  burette. 

Start  the  electrolysis  cold  with  a  current  of  NDioo  =  0.2  to 
0.3  volt.  After  about  an  hour,  increase  the  current  to  0.8-1.2 
ampere  and  heat  the  solution  to  50  to  60°.  If,  in  the  mean- 
time, a  brown  deposit  of  cobalt  peroxide  appears  on  the  anode, 
bring  it  back  into  solution  by  adding  about  0.1  gm.  of  hydroxyl- 
amine  sulphate  or  chloride. 

When,  as  a  result  of  the  removal  of  the  cobalt,  the  bath  has 
become  colorless,  add  a  few  drops  of  dilute  ammonium  hydroxide 
to  neutralize  the  mineral  acid  formed  during  the  electrolysis. 
In  this  way  the  last  traces  of  cobalt  are  precipitated. 

To  make  sure  that  all  the  cobalt  has  been  deposited  test  a 
little  of  the  solution  with  ammonium  hydroxide  and  sulphide. 
A  still  more  sensitive  test  is  the  reaction  with  ammonium  thio- 
cyanate,  amyl  alcohol  and  ether  (cf.  Treadwell-Hall,  Analytical 
Chemistry,  Vol.  I). 

*  Electrochemical  Analysis,  1905. 


LEAD  193 


GROUP  IV. 

Metals  which  may  be  deposited  on  the  cathode  or  may  be 
obtained  as  oxide  upon  the  anode. 

Lead. 

At.  Wt.  =  207.2.  Elec.  Equiv.  =  1.073  for  Pb++ ions.  Elec. 
Potential  =  +  0.148  volt.  Overvoltage  of  H2  =  0.35-0.64 
volt 

Lead  may  be  deposited  as  metal  upon  the  cathode  by  electro- 
lyzing  solutions  of  the  complex  oxalate,  the  acetate,  the  hydroxide, 
dissolved  in  caustic  alkali,  or  the  phosphate  dissolved  in  alkali 
hydroxide  or  in  phosphoric  acid,  sp.  gr.  1.7.  Two  difficulties  are 
encountered:  In  many  cases  a  little  peroxide  is  likely  to  form  on 
the  anode  and  the  deposited  lead  is  very  susceptible  to  slight 
oxidation.  None  of  the  many  methods  which  have  been  pro- 
posed for  the  quantitative  deposition  of  the  metal  upon  the 
cathode  is  quite  as  satisfactory  as  the  deposition  of  lead  peroxide, 
PbC>2,  upon  the  anode  from  a  nitric  acid  solution.  The  acid  con- 
centration may  be  made  so  high  that  no  copper  will  deposit  upon 
the  cathode  or  it  may  be  regulated  so  that  copper  is  deposited 
upon  the  cathode  while  lead  peroxide  is  being  formed  on  the 
anode. 

Two  explanations  have  been  suggested  to  account  for  the 
formation  of  the  lead  peroxide,  neither  of  which  is  entirely  sat- 
isfactory. According  to  Liebenow,  the  bivalent  lead  ions  are 
oxidized  to  negatively  charged  Pb02==  anions,  which  are  dis- 
charged at  the  anode.  Another  explanation  is  that  lead  tetra- 
nitrate  is  formed  by  anodic  oxidation  and  from  this  Pb02  is 
formed  by  hydrolysis.  It  is  not  easy  to  determine  whether 
the  mechanism  of  the  reaction  is  correctly  explained  by  either  of 
these  assumptions.  All  we  know  is  that  the  oxidation  takes 
place  at  the  anode  and  that  the  product  is  insoluble  in  nitric 
acid.  Rather  than  attempting  to  assign  hypothetical  stages  to 
this  process,  it  seems  simplest  to  express  the  reaction  as  follows: 

Pb++  +  2H20  +  20  ->Pb02  +  4H  + 


194          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

To  obtain  a  peroxide  deposit  adherent  to  the  electrode  and  in 
a  form  suitable  for  weighing,  Classen  recommends  the  use  of  an 
anode  with  a  dull,  unpolished  surface  and  uses  either  a  platinum 
dish,  the  inside  surface  of  which  has  been  roughened  by  the  sand 
blast,  or  a  gauze  electrode.  In  either  case  the  precipitate  has  a 
relatively  large  surface  upon  which  it  can  deposit. 

The  proper  conditions  for  the  deposition  of  lead  vary  according 
to  the  time  desired  to  spend  upon  the  analysis;  if  it  is  not  a 
question  of  speed,  the  electrolysis  may  well  be  allowed  to  proceed 
overnight  and  at  ordinary  temperatures.  If  it  is  desired  to 
hasten  the  process,  the  solution  should  be  heated.  The  strength 
of  the  current  and  the  amount  of  nitric  acid  are  regulated  accord- 
ingly. 

If  the*  analysis  is  to  be  made  at  room  temperature,  10  per  cent 
by  volume  *  of  nitric  acid  (sp.  gr.  1.35  to  1.38)  and  a  current 
density  of  NDi Oo  =  0.05  ampere  are  suitable.  If  the  work  is  carried 
out  at  60°  to  65°,  it  is  advisable  to  add  20  per  cent  by  volume  of 
the  nitric  acid  and  to  use  a  current  of  NDioo=  1.5  amperes.*  In 
the  latter  case  as  much  as  0.7  gm.  of  lead  peroxide  will  be  deposited 
in  less  than  an  hour;  the  precipitation  of  1.5  gms.  Pb02  requires 
about  3  hours. 

The  lead  peroxide  is  obtained  in  the  form  of  a  brownish-black 
coating  upon  the  anode.  A  test  is  made  to  see  if  the  analysis  is 
finished  by  mixing  about  20  cc.  of  water  with  the  electrolyte  and 
waiting  10  or  15  minutes  to  see  if  any  fresh  deposit  is  formed  upon 
the  newly  exposed  platinum  surface.  If  the  deposition  is  com- 
plete, the  peroxide  is  washed,  without  interrupting  the  current, 
using  nothing  but  water,  and  the  anode  is  dried  at  a  temperature 
of  220°. 

As  regards  the  exact  chemical  composition  of  the  lead  peroxide 
thus  obtained,  and  especially  as  regards  its  water  content  and  the 
proper  temperature  to  be  used  in  drying  the  deposit,  conflicting 
statements  have  been  made  by  different  authors.  Formerly, 
the  lead  peroxide  was  dried  at  180°.  Hollard  and  others  have 
found,  however,  that  the  peroxide  retains  a  little  water  when 
dried  at  this  temperature,  and  even  when  dried  at  220°  it  is 
not  to  be  regarded  as  perfectly  anhydrous.  To  compensate 
the  error,  Holland  recommends  that  the  weight  of  the  lead  in 

*  This  means  10  cc.  of  acid  in  100  cc.  of  solution. 

t  For  practice  about  1  gm.  of  lead  nitrate  should  be  used. 


LEAD  195 

the  precipitate  be  computed  by  multiplying  the  weight  of  per- 
oxide by  the  factor  0.853  instead  of  using  the  theoretical  factor 
0.8661.* 

The  experiments  of  A.  Vossen  in  the  Aachen  laboratory  con- 
firm the  fact  that  the  lead  peroxide  is  not  perfectly  anhydrous 
when  dried  for  an  hour  at  220°.  In  determining  small  quan- 
tities of  lead  up  to  about  0.1  gm.  PbO2,  the  factor  0.8658, 
which  is  practically  the  theoretical  value,  gave  correct  re- 
sults. For  larger  quantities  of  lead  peroxide  up  to  0.3  gm.,  the 
factor  0.  865,  and  for  quantities  over  0.3  gm.  the  factor  0.8635, 
was  found. 

The  oven  drying  at  a  high  temperature  and  the  use  of  an 
empirical  factor  may  be  avoided  by  cautiously  heating  the  per- 
oxide with  the  Bunsen  flame,  whereby  it  is  changed  into  yellow 
PbO;  from  this  the  lead  content  can  be  found  by  multiplying 
by  the  theoretical  factor,  0.9282. 

The  lead  monoxide  may  be  dissolved  readily  from  the  electrode, 
by  means  of  dilute  nitric  acid.  The  lead  peroxide  is  best  dis- 
solved from  the  platinum  by  placing  the  electrode  in  hot,  dilute 
nitric  acid  and  adding  a  little  of  reducing  agent,  such  as  oxalic 
acid,  sugar  or  alcohol. 

In  the  presence  of  phosphoric  acid  it  is  impossible  to  precipitate 
lead  quantitatively  as  peroxide  and  when  sufficient  phosphoric 
acid  is  present,  small  quantities  of  lead  may  be  deposited  quanti- 
tatively as  metal  upon  the  cathode,  f 

The  peroxide  deposition  is  also  incomplete  in  the  presence  of 
mercury,  arsenic  and  selenium. 

As  will  be  shown  in  the  part  of  this  book  which  is  devoted 
to  the  separation  of  the  metals  from  one  another,  the  deter- 
mination of  lead  as  peroxide  serves  to  effect  at  the  same  time 
a  separation  of  this  element  from  other  metals;  it  may  be 

*  Hollard  uses  the  factor  0.853  for  quantities  of  peroxide  weighing  less  than 
1  gm;  this  value  is  obtained  as  the  mean  of  a  great  many  experiments.  For 
quantities  of  lead  peroxide  weighing  between  1  gm.  and  1.5  gms.,  Hollard  found 
the  factor  0.857  to  give  correct  results.  These  factors  refer  to  deposits  dried 
at  200°.  The  high  weight  of  the  peroxide  is  not  due  to  the  presence  of  other 
oxides  of  lead.  F.  Lux  found  the  theoretical  content  of  PbO2  by  washing  the 
deposit  with  water  and  then  dissolving  it  in  a  mixture  of  dilute  nitric  acid 
and  a  known  quantity  of  oxalic  acid,  finally  titrating  the  excess  of  the  latter 
with  potassium-permanganate  solution. 

t  A.  L.  Linn.,  J.  Am.  Chem.  Soc.,  24,  435  (1902). 


196 


QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 


RAPID   DEPOSITION   OF  LEAD   PEROXIDE   IN   NITRIC-ACID 
SOLUTION.* 


Experiments  performed  by 

Exner. 

A.  Fischer 
and  Boddaert 
at  Aachen. 

R.O.Smith. 

H.J.S.Sand. 

A.  Fischer 
at  Aachen. 

Electrode 

Dish  and 
rotating 
spiral 

20  cc. 
HNO3 
(sp.4g, 

125  cc. 

0.26-1.1 
gm.  as 
nitrate 

Hot 
4.5  volts 

10  amp. 

600 
10  to  13 

Dish  and 
rotating 
disk 

Dish  and 
rotating 
spiral 

25  cc. 
HN03 

(sp.  gr. 
1.4) 

125  cc. 

0.06-0.58 
gm.  as 
nitrate 

95° 

3.6  to  3.8 

volts 

10  to  11 
amp. 

800 
15 

Sand's 
elec- 
trodes 

15  cc. 
HNO3 

(sp.  gr. 
1.4) 

85  cc. 

0.13-0.14 
gm.  as 
nitrate 

60° 

2.2  to  2.4 
volts 

Dish  and 
rotating 
disk 

20  cc. 
HNO3 

(sp.  gr. 
1.4) 

125  cc. 

0.29  gm. 
as  ni- 
trate 

€0°to65° 

1.9  to  2.2 
volts 

Electrolyte  contained 
Volume 

125  cc. 

0.47  gm.  as 
nitrate 

95° 

3.6  to  3.8 

volts 

10  to  11 
amp. 

800 
15 

Quantity  of  metal.  .  .  . 
Temperature  

Potential 

NDioo  

Revolutions  

800 
7  to  10 

800 
24 

Time  in  minutes  

mentioned  here,  however,  that  in  the  presence  of  silver  or 
bismuth  the  lead  peroxide  will  be  contaminated  with  a  little 
of  these  metals.  Chlorine,  selenium,  mercury,  phosphorus  and 
arsenic  compounds  must  not  be  present  in  the  solution 
analyzed.  In  the  presence  of  very  little  manganese,  the 
method  gives  satisfactory  results,  but  it  is  then  necessary  to 
use  a  considerable  excess  of  nitric  acid  (about  30  cc.)  to 
carry  out  the  analysis  in  a  hot  solution  (70°),  and  to  use  a 
fairly  strong  current  (up  to  2  amperes)  so  that  the  deposition 
will  take  place  quickly  with  but  slight  reduction  of  the  nitric 
acid  to  ammonia,  f 

*  General  references  concerning  the  rapid  electrodeposition  of  lead  and 
the  separation  of  this  metal  in  different  solutions:  Exner,  J.  Am.  Chem. 
Soc.,  26,  896  (1903).  A.  Fischer  and  Boddaert,  Z.  Elektrochem,  10,  945 
(1904).  R.  O.  Smith,  Thesis,  U.  Pa.,  1905.  H.  J.  S.  Sand,  J.  Chem.  Soc., 
London,  91,373(1907). 

t  B.  Neumann,  Chem.-Ztg.,  20,  381  (1896). 


MANGANESE  197 

The  fact  that  lead  cannot  be  deposited  as  metal  from  a  solution 
distinctly  acid  with  nitric  acid  can  be  explained  readily.  To  pre- 
vent deposition  of  lead  as  metal  on  the  cathode,  the  potential  of 
the  cathode  must  be  kept  constantly  below  the  discharge  potential 
of  lead  ions.  This  is  accomplished  by  providing  ions  which  are 
more  readily  discharged  than  are  the  lead  ions.  In  acid  solutions, 
the  hydrogen  ions  from  nitric  acid  fulfil  this  requirement  and  their 
discharge  is  easier  if  the  concentration  of  the  acid  is  fairly  high; 
hence  the  addition  of  an  excess  of  nitric  acid.  *  Luckow  and  others 
have  discovered  that  the  lead-peroxide  deposit  is  particularly  good 
when  copper  ions  are  present  in  the  solution  and  this  is  explained 
by  the  fact  that  copper  ions  are  discharged  even  more  easily  than 
hydrogen  ions.  Thus,  some  authorities  recommend  the  addition 
of  a  little  copper  nitrate  to  the  solution  of  lead  nitrate. 

Concerning  the  determination  of  lead  in  lead  sulphate,  consult 
page  231. 

Manganese. 

At.  Wt.  =  54.93.     Elec.    Equiv.  =  0.285   mg.    for   Mn  +  +  ions. 
Elec.  Potential  =  +  1.075  volt. 

It  is  practically  out  of  the  question  to  attempt  to  deposit 
manganese  as  metal  upon  the  cathode,  except  perhaps  in  the 
form  of  an  amalgam.  It  is  more  difficult  to  obtain  satisfactory 
deposits  of  manganese  dioxide  upon  the  anode  than  in  the  case 
of  lead.  With  lead,  for  example,  a  large  excess  of  nitric  acid  can 
be  employed,  but  with  manganese  the  oxidation  is  likely  to  go 
too  far,  when  much  nitric  acid  is  present,  and  a  soluble  perman- 
ganate results.  Many  methods  have  been  proposed,  but  very 
few  have  given  entire  satisfaction.  Moreover,  the  chief  problem 
in  the  analytical  chemistry  of  manganese  is  the  separation  of 
this  element  from  others  and  the  electrolytic  method  is  suitable 
for  such  separations  only  in  special  cases. 

Manganese  sulphate  and  manganese  nitrate  are  suitable  for  the 
electrolysis  but  manganese  chloride  is  not.  The  solution,  con- 
taining 0.2  to  0.25  gm.  of  manganese,  is  treated  with  1.5  to  2  gms. 
of  chrome  alum  and  10  gms.  of  sodium  acetate,  diluted  to  about 
125  cc.  and  electrolyzed  at  80°  in  a  dish  with  sand-blasted  inner 

*  Concerning  the  decomposition  of  nitric  acid  itself,  see  p.  118. 


198  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

surface  using  a  current  density  of  NDioo  =  0.6  to  1  ampere  and 
voltage  of  2.8  to  4  volts;  in  this  case,  the  dish  serves  as  anode.  The 
electrolysis  requires  about  an  hour  and  a  quarter.  After  an  hour 
or  so  has  elapsed,  some  water  is  added  to  raise  the  surface  of  the 
liquid  and  it  is  noted  whether  any  further  deposit  is  formed  (cf. 
p.  194). 

A  safer  way  is  to  test  a  portion  of  the  solution  with  lead  peroxide 
(free  from  manganese!)  and  nitric  acid;  a  mere  trace  of  manganese 
will  show  the  permanganate  color. 

After  breaking  the  circuit,  the  liquid  is  poured  out  of  the  dish, 
the  deposit  is  washed  repeatedly  with  water,  and  is  changed  finally 
into  mangano-manganic  oxide,  Mn304,  by  ignition  over  the  blast 
lamp.  When  the  transformation  is  complete  the  platinum  will 
be  covered  with  a  uniform  reddish-brown  deposit.  After  igniting 
and  cooling  it  is  well  to  rinse  again  with  hot  water,  in  order  to 
remove  traces  of  impurity  deposited  with  the  manganese  dioxide 
by  the  current.  After  this  final  washing,  the  dish  is  ignfted  again 
and  cooled  in  a  desiccator. 

In  this  determination  the  use  of  a  platinum  dish  as  anode  is 
necessary  because  it  will  not  do  to  ignite  a  platinum  gauze  elec- 
trode in  the  flame  of  the  blast  lamp;  changes  are  likely  to  result 
in  the  composition  of  the  oxide  and  losses  due  to  the  formation  of 
fine  powder.  Engel  found  that  after  the  ignition  of  his  platinum 
dishes  the  weight  was  diminished  about  one  milligram.  He  con- 
cluded that  the  loss  took  place  during  the  electrolysis  and  not 
during  the  solution  of  the  deposit  (in  sulphuric  acid  and  hydrogen 
peroxide).  Judging  from  the  experience  of  other  chemists,  how- 
ever, it  seems  probable  that  the  loss  was  due  to  volatilization  of 
some  constituent  of  the  dish  during  the  heating  over  the  blast 
lamp;  it  is  a  common  experience  to  find  a  platinum  crucible  that 
will  steadily  lose  weight  on  being  heated  over  the  blast  lamp 
while  another  crucible  under  similar  treatment  does  not  show  such 
loss.  In  the  case  of  the  electrolytic  determination  of  manganese, 
therefore,  the  weight  of  the  platinum  dish  should  be  determined 
after  the  deposit  has  been  removed,  rather  than  before  start- 
ing the  electrolysis.  J.  Koster  *  recommends  the  use  of  platinum- 
iridium  ware;  which  does  not  experience  more  than  0.2  mgm. 
loss  by  heating  over  the  blast  lamp. 

*  Z.  Elektrochem.,  10,  553  (1904). 


MANGANESE  199 

As  regards  the  original  deposit  upon  the  anode,  numerous  ex- 
periments have  shown  beyond  doubt  that  manganese  dioxide 
is  never  deposited  entirely  as  such.  According  to  the  conditions 
of  the  experiment,  the  black  deposit  contains  varying  quan- 
tities of  water  as  well  as  of  oxygen  and  must  be  regarded  as 
consisting  of  a  mixture  of  manganese  dioxide  and  lower  oxides  in 
a  hydrated  condition;  the  only  way  to  obtain  a  constant  weight 
is  to  convert  it  into  an  oxide  of  more  constant  composition  by 
igniting  it. 

Although  the  composition  of  the  deposit  is  not  of  prime  impor- 
tance, the  nature  of  the  deposit  is,  nevertheless,  of  considerable 
moment;  it  must  adhere  so  firmly  to  the  platinum  that  it  is  not 
loosened  during  the  washing  and  it  must  not  be  so  brittle  that  it 
will  scale  off  during  the  igniting.  These  necessary  properties  are 
obtained  in  Engel's  method  by  the  addition  of  chrome  alum  to 
the  electrolyte  containing  the  manganous  salt  and  sodium  acetate. 
The  favorable  effect  of  chrome  alum  upon  the  nature  of  the  man- 
ganese deposit,  is  explained  by  Engel  *  as  follows : 

If  one  has  in  mind  the  disturbing  effect  that  the  evolution  of 
hydrogen  has  upon  the  nature  of  the  cathode  deposit  (cf.  p.  80), 
it  seems  reasonable  that  the  anode  deposit  would  be  loosened  in 
a  similar  manner  by  evolution  of  oxygen.  The  favorable  effect 
of  the  chromic  salt,  therefore,  may  be  due  to  the  fact  that  it  is 
oxidized  at  the  anode  and  thus  prevents  the  evolution  of  oxygen 
gas.  As  a  matter  of  fact,  chromate  is  formed  during  the  elec- 
trolysis, but,  on  the  other  hand,  the  fact  that  a  deposit,  which 
scales  off,  is  formed  in  the  absence  of  chrome  alum,  when  the  elec- 
trolysis is  carried  out  at  a  potential  below  the  decomposition-poten- 
tial of  water,  is  contrary  to  such  an  assumption.  The  mechanical 
effect  of  free  oxygen,  therefore,  cannot  be  the  cause  of  the  bad 
adherence  of  the  manganese  peroxide  to  the  anode.  It  appears 
more  likely  that  a  moderate  evolution  of  oxygen  is  necessary  to 
give  the  deposit  a  porous,  pulverulent  nature  in  virtue  of  which  it 
will  adhere  so  firmly  to  the  anode  that  it  will  not  be  detached 
during  the  washing  and  will  not  spring  away  from  the  platinum 
during  the  ignition.  In  fact  Engel  assumes  that  the  oxygen  exerts 
a  chemical  effect.  He  believes  that  the  desirable  properties  of 
the  manganese-oxide  deposit  are  due  to  the  admixture  of  the 

*Z.  Elektrochem.,  2,  413  (1895);  3,  286,  305  (1896). 


200 


QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 


brittle  peroxide  with  a  pulverulent  oxide.  The  mass  thus  formed 
is  of  such  a  nature  that  it  will  not  scale  off  during  the  heating  and 
has  the  property  of  adhering  firmly  to  the  platinum. 

The  part  played  by  the  oxygen,  which  Engel  regards  as  a  reduc- 
ing effect  (just  as  hydrogen  peroxide  may  cause  a  reduction),  is  to 
reduce  a  part  of  the  peroxide,  deposited  by  the  strong  current,  to 
oxide  while  the  chrome  alum  takes  care  of  any  excess  of  oxygen 
and  thus  prevents  it  from  exerting  a  harmful  effect  upon  the 
nature  of  the  anode  deposit.  If,  however,  the  current  strength  is 
not  sufficient  to  form  the  necessary  oxygen  to  act  upon  the  man- 
ganese peroxide,  then  the  chromic  salt  itself  serves  to  reduce  a 
part  of  the  peroxide  and  supports  the  action  of  the  oxygen.  Thus 
the  chrome  alum  plays  the  part  of  a  regulator:  if  the  evolution  of 
oxygen  is  too  strong,  it  unites  with  a  part  of  the  oxygen  to  form 
chromate,  and  if  the  oxygen  evolution  is  not  strong  enough,  it 
serves  to  reduce  a  part  of  the  peroxide.  This  reduction  takes  place 
as  experiments  have  shown,  in  a  solution  containing  ammonium 
acetate,  chrome  alum  and  manganese  peroxide,  only  when  the 
temperature  reaches  80°;  hence  the  necessity  of  heating  the  elec- 
trolyte to  this  temperature. 

RAPID  DEPOSITION  OF  MANGANESE  PEROXIDE  IN  ACETATE 

SOLUTION.* 


Experiments  performed  by 
J.  Koster  at  Aachen. 

Electrode                 

Dish  and  rotating  disk 

Electrolyte  contained  

10  gms.  of  ammonium  acetate 

i 
Volume                         

2  to  3  gms.  of  chrome  alum 
10  cc.  of  alcohol 

110  to  130  cc 

Quantity  of  metal  

0.3  gm.  as  manganous  ammo- 

Temperature   

nium  sulphate 
75°  to  80° 

Potential  of  the  bath 

7  volts 

Current  strength              

4  to  4  5  amperes 

Revolutions  

600  to  700 

Time  

20  to  25  minutes 

*  References  on  the  rapid  electrolytic  estimation  of  manganese.    Exner,  J. 
Am.  Chem.  Soc.,  26,  896  (1903).    Koster,  Z.  Elektrochem.,  10,  553  (1904). 


URANIUM  201 

The  assumption  by  Engel  of  the  reducing  effect  of  the  oxygen  is 
not  absolutely  necessary,  for  the  reducing  action  of  the  chrome 
alum  is  of  itself  sufficient  to  account  for  the  same  effect. 

Koster,  in  the  author's  laboratory,  has  worked  out  the  condi- 
tions for  the  rapid  deposition  of  manganese  peroxide. 

Deposition  of  Manganese  from  Formic-acid  Solution. 

G.  P.  Scholl,*  in  studying  the  behavior  of  manganese  salts  in 
the  presence  of  formate  and  free  formic  acid,  found  that  the  best 
results  were  obtained  when  formic  acid  alone  was  added.  Since 
formic  acid  is  a  poor  conductor  of  the  current,  it  is  necessary 
to  use  a  current  of  unusually  high  voltage  in  order  to  get  the 
requisite  current 'strength.  The  difficulty  is  less  serious,  however, 
if  a  sieve  electrode  is  used  instead  of  the  disk  or  spiral.  The  elec- 
trode used  by  Scholl  had  the  same  shape  as  the  platinum  dish 
which  is  used  as  anode  but  it  is  a  little  smaller.  It  is  perforated 
like  a  sieve  and  has  about  60  sq.  cm.  of  surface  (Fig.  22,  p.  57). 
The  solution,  containing  0.1  to  0.2  gm.  of  manganese  as  sulphate, 
is  treated  with  5  cc.  of  formic  acid  (sp.  gr.  1.09)  and  electrolyzed 
at  the  laboratory  temperature  with  a  current  density  of  NDioo  = 
0.8  to  1.0  ampere  and  at  a  final  potential  of  about  7  volts;  the 
time  required  is  from  3  to  5  hours.  Deposits  yielding  as  much  as 
0.288  gm.  of  Mn304  after  ignition  are  found  to  adhere  well  to 
the  electrode.  This  is  not  possible  by  the  acetate  method  (p.  166). 
Scholl  noticed  no  decrease  in  the  weight  of  the  platinum  dish. 

Uranium. 

At.  Wt.  =  238.2.     Elec.  Equiv.  =  1.235  mg.  for  U02  +  +  ions. 

Uranium  is  deposited  upon  the  cathode  in  the  form  of  oxide 
from  solutions  of  the  acetate,  sulphate  or  nitrate.  The  deposit 
is  yellow  at  first  and  consists  of  uranyl  hydroxide,  but  during  the 
progress  of  the  electrolysis  it  assumes  a  darker  hue.  When  the 
solution  has  become  colorless,  a  little  of  it  is  tested  with  potassium 
ferrocyanide  or  with  ammonium  sulphide. 

The  current  is  turned  off,  the  deposit  is  washed  first  with  water 
containing  some  acetic  acid,  then  with  hot  water,  and  the  oxide  is 
converted  by  ignition  into  urano-uranic  oxide,  UaOg.  If,  during 

*  J.  Am.  Chem.  Soc.,  25,  1045  (1903). 


202  QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS 

the  washing,  a  little  of  the  deposit  is  washed  off  the  dish,  it  may 
be  collected  upon  a  washed  filter,  placed  in  the  dish  and  ignited. 

L.  Kollock  and  E.  F.  Smith  *  recommend  the  following  condi- 
tions: To  the  solution  containing  0.1  to  0.23  gm.  of  U3O8  in  the 
form  of  uranyl  acetate,  0.2  cc.  of  29  per  cent  acetic  acid  is  added, 
and  after  diluting  to  125  cc.  the  solution  is  heated  to  70°  and 
electrolyzed.  The  current  density  may  lie  between  NDioo  =  0.28 
and  0.065  ampere  and  the  corresponding  potential  is  16.25  to  4.25 
volts.  The  duration  of  the  analysis  is  between  5  and  6  hours  in 
either  case. 

A  uranyl-nitrate  solution  containing  0.12  gm.  of  U3O8  in  125  cc. 
was  electrolyzed  at  75°  with  a  current  density  of  NDi^  =  0.019  to 
0.038  ampere  at  2.25  to  4.6  volts;  the  electrolysis  required  between 
5.5  and  7.75  hours. 

For  uranyl-sulphate  solutions,  containing  0.13  to  0.14  gm.  of 
U3O8  in  125  cc.,  the  conditions  recommended  are:  75°,  NDi00  = 
0.019  to  0.038  ampere,  2  to  2.25  volts,  5  to  7  hours. 

Inasmuch  as  the  separation  of  uranium  from  certain  other  metals 
offers  considerable  difficulty  by  other  methods  of  analysis,  the 
electrolytic  method  often  proves  serviceable. 

Thallium. 

At.  Wt.  =  204.0.     Elec.  Equiv.  =  1.056  mg.  for  Tl  +  +  ions. 

The  metal  thallium  resembles  lead  closely  in  its  chemical  be- 
havior and  like  the  latter  can  be  deposited  as  metal  upon  the 
cathode;  on  coming  in  contact  with  the  air,  however,  it  is  oxidized 
so  rapidly  that  the  deposit  cannot  be  weighed  accurately.  G.  Neu- 
mann,! while  working  at  Aachen,  devised  an  indirect  method  for 
determining  thallium;  an  ammonium  oxalate  solution  of  the  metal 
was  electrolyzed  out  of  contact  with  the  air  and  the  volume  of 
hydrogen  set  free  on  dissolving  the  deposit  in  hydrochloric  acid 
was  determined. 

Determination  of  Thallium  as  Oxide. 

The  conditions  under  which  thallium  may  be  deposited  as 
T12O3  upon  the  anode  were  established  by  J.  E.  Heiberg.J  From 
0.2  to  1.0  gm.  of  thallous  sulphate  (a  compound  into  which  it  is 

*  J.  Am.  Chem.  Soc.,  23,  607  (1901). 

fBer.,  21,  356  (1888). 

j  Z.  anorg.  Chem.,  35,  347  (1903). 


THALLIUM  203 

easy  to  convert  other  thallous  as  well  as  thallic  compounds)  is 
dissolved  in  80  to  100  cc.  of  water  in  a  roughened  platinum  dish. 
The  solution,  after  being  treated  with  3  to  6  cc.  of  normal  sulphuric 
acid  and  5  to  10  cc.  of  acetone,  is  electrolyzed  at  a  potential  of 
1.7  to  2.3  volts,  using  the  dish  as  anode.  Toward  the  end  of  the 
electrolysis,  the  potential  may  be  raised  to  2.5  volts,  provided  there 
is  not  a  strong  evolution  of  oxygen,  which  would  tend  to  loosen  the 
deposit.  The  current  strength  in  the  poorly  conducting  solution 
is  only  0.02  to  0.05  ampere.  The  temperature  must  lie  between 
50°  and  55°  and  the  water  lost  by  evaporation  must  be  replaced. 

Potassium  iodide  precipitates  pale  yellow  thallous  iodide  from 
very  dilute  thallium  solutions  and  the  precipitate  is  very  insolu- 
ble in  an  excess  of  potassium-iodide  solution.  When,  therefore, 
0.5  cc,  of  the  solution  does  not  give  more  than  a  trace  of  opalescence 
on  being  treated  with  5  cc.  of  10  per  cent  potassium-iodide  solution, 
the  electrolysis  may  be  regarded  as  finished.  The  dish  is  then 
emptied  quickly,  rinsed  successively  with  water,  alcohol  and 
ether,  and  dried  for  20  minutes  at  160°  to  165°.  The  brown  coat- 
ing consists  of  thallium  sesquioxide,  T1203.  From  7  to  10  hours 
are  required  for  the  deposition  of  0.5  gm.  of  this  oxide,  corrrespond- 
ing  to  0.55  gm.  of  sulphate. 

The  conductivity  of  the  solution  can  be  increased  by  adding  to 
the  bath  one  or  two  grams  of  alkali  sulphate  but  the  deposit  must 
be  thoroughly  washed  or  the  results  will  be  too  high. 

The  prescribed  acidity  suffices  to  prevent  the  precipitation  of 
hydroxide  during  the  electrolysis  but  it  is  not  sufficient  to  prevent 
any  deposition  of  metallic  thallium  upon  the  cathode.  This  does 
no  harm,  however,  as  the  deposit  redissolves  during  the  progress 
of  the  electrolysis  and  it  is  better  to  work  with  the  above-men- 
tioned quantity  of  acid  rather  than  to  attempt  to  prevent  any  depo- 
sition of  metal  by  increasing  the  acidity,  because  in  the  latter  case 
the  deposition  of  oxide  on  the  anode  is  likely  to  be  incomplete. 

It  would  be  possible  to  prevent  deposition  of  metallic  thallium 
by  keeping  the  voltage  of  the  current  below  the  decomposition 
potential  of  thallium,  but  this  would  result  in  making  the  current 
so  weak  that  it  would  take  too  long  to  carry  out  the  determination. 

The  acetone  exerts  a  favorable  effect  upon  the  physical  nature 
of  the  deposit  but  it  is  not  yet  quite  clear  why  it  does.  Since 
the  acetone  is  gradually  decomposed  by  the  action  of  the  electric 
current,  it  is  necessary  to  add  enough  at  the  start  so  that  some  of 


204          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

it  will  remain  to  the  end  of  the  process.  The  author  has  found 
that  10  cc.  of  acetone  are  sufficient  for  an  experiment  lasting 
17  hours. 

The  evolution  of  oxygen  caused  by  too  high  an  electromotive 
force  is  also  noticed  when  the  electrolysis  is  carried  out  at  high 
temperatures  but  the  temperature  most  favorable  for  the  deter- 
mination has  been  found  to  lie  between  50°  and  55°.  At  this 
temperature  there  is  considerable  evaporation  during  the  elec- 
trolysis and  there  is  danger  of  the  upper  parts  of  the  deposit 
becoming  so  dry  that  upon  the  addition  of  water  the  thin  layer  of 
oxide  will  be  loosened  from  the  platinum.  To  prevent  this,  it  is 
advisable  to  allow  water  to  constantly  drop  into  the  solution 
instead  of  adding  it  in  larger  quantities  intermittently.  Under 
the  conditions  given  above,  a  beautiful  brown  coating  of  oxide 
which  adheres  well  to  the  platinum  is  obtained. 

In  drying  the  deposit,  care  should  be  taken  not  to  let  it  come 
in  contact  with  a  free  flame,  as  this  may  introduce  a  positive 
error  in  the  weight  obtained  (due  to  SO3,  etc.). 

Thallium  oxide,  not  being  a  peroxide,  dissolves  in  hydrochloric 
acid  without  evolution  of  chlorine,  and  thus  this  acid  may  be 
used  for  dissolving  the  deposit  from  the  platinum. 

The  deposition  of  thallium  oxide  will  undoubtedly  be  hastened 
by  stirring  the  electrolyte  and  using  a  stronger  current. 

Chromium. 
At.  Wt.  =  52.0.     Elec.  Equiv.  =  0.18  mg.  for  Cr  +  +  +  ions. 

Electrolysis  may  serve  in  two  ways  for  the  quantitative  deter- 
mination of  chromium:  chromic  ions  may  be  converted  into 
chromate  ions  by  oxidation  at  the  anode,  after  which  it  is  necessary 
to  determine  the  latter  by  one  of  the  usual  methods  of  quantitative 
analysis;  or,  the  chromium  may  be  converted  into  mercury 
amalgam  by  using  a  mercury  cathode.  Both  methods  are  of 
value  only  in  effecting  certain  separations. 

Oxidation  of  Chromic  Salt  to  Chromate. 

Ammonium-chromium  oxalate  is  converted  into  ammonium 
chromate  by  the  action  of  the  electric  current.  The  method  will 
be  explained  more  fully  in  Part  III  of  this  book. 

The  oxidation  may  be  accelerated  by  maintaining  the  following 
conditions. 


CHROMIUM  205 

RAPID  OXIDATION  OF  CHROMIC  SALT  TO  CHROMATE.* 


Experiments  performed  by 
A.  Fischer  at  Aachen. 


Electrode 

Electrolyte  contained .  . . 

Volume 

Quantity  of  metal 

Temperature 

Potential 

Current  strength 

Revolutions 

Time.. 


Dish  and  rotating  disk 
15  gms.  of  ammonium  oxalate 

120  cc. 
0.14  gm.  as  chloride  or  sulphate 

80° 

5  to  7  volts 

5.8  to  5.4  amperes 

600  per  minute 

90  minutes 


Determination  of  Chromium  as  Chrome  Amalgam. 
R.  E.  Myers  f  used  as  electrolyzing  vessel  the  beaker  (Fig.  49) 
first  proposed  by  E.  F.  Smith.t     It  is  about  8.5  cm.  tall  and  3.5 
cm.  in  diameter  with  a  platinum  wire  fused  into  the  bottom,  or  into 
the  walls  near  the  bottom.     This  wire  is  covered  on  the  inside  of  the 
beaker  with  a  layer  of  mercury,  and  is  bent  under  the  bottom  of 
the  beaker  in  such  a  way  that  when  it  is  placed  upon  a  copper 
disk,  which  is  connected  with  the  negative  pole  of  a  source  of 
electricity,  the  mercury  will  serve  as  cathode.     A  strip  of  platinum 
foil,  or  a  platinum  wire  wound  into  a  spiral,  may  be  used  as  anode. 
About  70  gms.  of  mercury  are  placed  in  the  beaker  and  the  total 
weight  of  glass  and  mercury  is  determined.     To  make  sure  that 
the  weighing  takes  place  under  precisely  the  same 
conditions  as  at  the  end  of  the  experiment,  it  is 
washed  successively  with  water,  alcohol  and  ether 
in  the  same  way  that  the  amalgam  is  to  be  washed. 
This  is  done  by  filling  the  dish  about  one  third  full 
of  water,  and  whirling  it  round  with  the  beaker  in- 
clined so  that  all  of  the  walls  are  rinsed.     The  same 
operation  is  repeated  with  alcohol  and  finally  with 
ether.      After  the  odor  of  ether  has  disappeared, 
the  outside  of  the  beaker  is  wiped  with  a  cloth,  and, 

*  General  references  to  the  literature  on  the  rapid  electrolytic  deposition 
of  chromium  in  different  solutions:  E.  F.  Smith  and  Kollock,  J.  Am.  Chem. 
Soc.,  27,  1255  (1904).  A.  Fischer,  Chem.-Ztg.,  31,  25  (1907). 

t  J.  Am.  Chem.  Soc.,  26,  1124  (1904).  J  Ibid.,  26,  887  (1903). 


FIG.  49. 


206  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

after  standing  fifteen  minutes  in  a  desiccator,  the  beaker  and  its 
contents  are  weighed. 

The  solution  of  chromic  sulphate  containing  0.1  to  0.2  gm.  of 
chromium  is  poured  into  the  weighed  beaker  and,  after  acidifying 
with  3  or  4  drops  of  concentrated  sulphuric  acid,  it  is  electrolyzed 
with  an  initial  potential  of  7  to  7.5  volts,  corresponding  to  a  current 
of  0.3  to  0.4  ampere.  On  account  of  the  increase  of  acid  in  the 
bath  as  a  result  of  the  electrolysis,  the  potential  falls  gradually 
to  5.5  or  6  volts  and  the  current  increases  to  0.55  or  0.7  ampere. 
The  electrolysis  requires  about  14  hours  and  can  be  carried  out 
conveniently  overnight. 

The  end  of  the  process  can  be  determined  by  making  a  little 
of  the  solution  alkaline  with  caustic  potash,  adding  hydrogen 
peroxide  and  acidifying  with  sulphuric  acid.  If  a  trace  of  chro- 
mium is  present,  the  blue  color  of  perchromic  acid  is  obtained. 

At  the  end,  the  mercury  is  washed  with  water  without  breaking 
the  circuit  until  the  pointer  of  the  ammeter  points  nearly  to  the 
zero  mark.  The  washing  is  finished  in  the  way  described  on  the 
previous  page  except  that  each  liquid  is  added  several  times. 

Chrome-amalgam  is  decomposed  by  water,  setting  free  black, 
pulverulent  chromium;  for  this  reason  the  washing  must  be  done 
as  quickly  as  possible.  The  decomposition  of  the  amalgam  takes 
place  more  readily  in  proportion  to  the  quantity  of  chromium  in  it 
and  for  this  reason  it  is  not  advisable  to  deposit  more  than  0.2  gm. 
of  chromium  with  70  gms.  of  mercury.  Such  a  quantity  of  mer- 
cury should  therefore  be  used  for  but  a  single  analysis 

Molybdenum. 

At.  Wt.  =  96.0.    Elec.  Equiv.  =  0.166  mg.  for  hexavalent  Mo. 

Molybdenum  belongs  to  the  class  of  metals  which,  up  to  the 
present  time,  have  only  been  obtained  as  oxide  upon  the  cathode.* 

According  to  L.  G.  Kollock  and  E.  F.  Smith,  f  the  molybdenum 
in  sodium  molybdate,  a  salt  into  which  it  is  easy  to  convert 
molybdenum  (cf.  p.  208),  may  be  determined  in  the  following 
manner: 

The  aqueous  solution  of  the  salt,  containing  0.13  to  0.26  gm. 
of  MoOa,  is  acidified  with  0.1  to  0.2  cc.  of  concentrated  sulphuric 

*R.  E.  Myers  used  a  mercury  cathode  and  determined  the  molybdenum 
as  amalgam,  J.  Am.  Chem.  Soc.,  26,  1124  (1904). 

t  J.  Am.  Chem,  Soc,,  23,  669  (1901). 


MOLYBDENUM  207 

acid,  diluted  to  125  cc.,  heated  to  about  75°  and  electrolyzed  with 
a  current  of  NDioo  =  0.02  to  0.04  ampere  at  about  2  volts  potential. 
The  solution  assumes  a  deep  blue  color  which  gradually  disappears. 

The  electrolysis  requires  from  2.5  to  7  hours,  according  to  the 
quantity  of  molybdenum  present,  and  the  black,  lustrous,  firmly 
adherent  coating]  upon  the  cathode  consists  of  hydrated  molyb- 
denum sesquioxide,  Mo2O3.a:  H2O.  The  precipitation  is  complete 
when  a  little  of  the  solution,  after  the  addition  of  hydrochloric 
acid,  ammonium  thiocyanate  and  a  little  zinc,  no  longer  shows  the 
red  color  of  'molybdenum  thiocyanate.*  The  deposited  oxide  is 
washed  without  breaking  the  circuit. 

The  black  hydrated  sesquioxide  cannot  be  dried  to  constant 
weight.  The  moist  deposit,  therefore,  is  dissolved  in  dilute  nitric 
acid,  the  solution  evaporated  to  dryness  and  heated  on  an  iron 
plate  until  the  nitric  acid  is  all  expelled.  If  the  residue  should 
be  colored  blue  by  reduction,  it  is  heated  again  with  nitric  acid. 
The  residual  white  molybdic  acid  H2MoO4  is  weighed. 

A.  Chilesotti  and  A.  Rozzi  f  have  found  that  the  molybdenum 
precipitate  obtained  in  this  way  may  contain  alkali  and  that  the 
alkali  content  is  made  larger  on  increasing  the  quantity  of  alkali 
present  in  the  solution  and  is  lessened  by  increasing  the  quantity 
of  free  sulphuric  acid.  If  the  content  of  alkali  salt  (e.g.,  K2SO4) 
does  not  exceed  0.75  per  cent,  then  the  error  may  be  compensated 
by  the  addition  of  0.4  to  0.5  per  cent  of  free  sulphuric  acid  to  the 
electrolyte.  With  higher  alkali  content,  as  obtained  by  the  fusion 
of  a  molybdenum  ore  with  sodium  carbonate  (cf.  p.  208),  it  is 
necessary,  these  authors  claim,  to  dissolve  the  deposited  oxide, 
after  washing  it  in  the  usual  way,  in  nitric  acid  and,  after  removing 
the  excess  of  the  latter  by  evaporation,  to  dissolve  the  residual 
molybdic  acid  in  ammonia.  The  resulting  solution  is  treated 
with  enough  sulphuric  acid  so  that  finally  0.4  to  0.5  per  cent  of 
free  acid  is  present,  and  this  solution  is  again  electrolyzed.  The 
deposition  of  the  molybdenum  from  the  ammonium-molybdate 
solution  is  quantitative  if  the  free  sulphuric-acid  content  lies 
between  0.5  and  0.05  per  cent. 

The  method  is  suitable  for  the  separation  of  molybdenum  from 
the  alkalies. 

*  The  color  disappears  upon  the  addition  of  phosphoric  acid  (difference 
from  iron). 

t  Z.  Elektrochem.,  11,  879  (1905). 


208 


QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 


Exner  obtained  a  rapid  electrolytic  determination  of  molyb- 
denum under  the  following  conditions. 

RAPID   ELECTROLYTIC  DEPOSITION   OF  MOLYBDENUM   SES- 

QUIOXIDE. 


Experiment  performed  by  F.  Exner. 


Electrode 

Electrolyte  contained 

Volume  

Quantity  of  metal 

Temperature 

Potential 

Current  strength 

Revolutions 

Time . . 


Dish  and  rotating  spiral 
2  cc.  H2SO4   (1  :  10)  and  1  gm.  K2S04 

120  cc. 
0.23  gm.  as  MoO3 

Hot 

16  volts 

5  amp. 

300  to  400 

20  min. 


Analysis  of  Molybdenite. 

To  determine  electrolytically  the  quantity  of  molybdenum 
present  in  the  mineral  molybdenite,  0.14  to  0.28  of  the  fine  powder 
is  fused  with  a  mixture  of  sodium  carbonate  and  sodium  nitrate, 
the  melt  is  extracted  with  water  and  filtered.  The  resulting 
aqueous  solution  is  acidified  with  acetic  acid,  the  carbon  dioxide  is 
all  expelled  by  boiling,  and,  after  diluting  to  125  cc.,  the  acetic- 
acid  solution  is  electrolyzed  at  85°  with  a  current  of  NDioo  =  0.07 
at  4.4  volts.  The  electrolysis  requires  longer  in  an  acetic-acid 
solution  than  in  one  of  sulphuric  acid. 

After  the  solution  has  been  freed  from  molybdenum,  the  sul- 
phuric acid  is  determined  by  precipitation  with  barium  chloride. 

If  it  is  not  desired  to  determine  the  sulphur  as  well  as  the 
molybdenum,  it  is  better  to  acidify  the  aqueous  extract  of  the 
melt  with  sulphuric  acid  rather  than  with  acetic  acid. 

Vanadium. 

P.  Truchot*  has  proposed  a  method  for  the  electrolytic  determi- 
nation of  vanadium  whereby  this  element  is  obtained  as  hydrated 
oxide  upon  the  electrode.     The  method  succeeds  only  when  the 
quantity  of  vanadium  present  is  not  more  than  0.25  gm.  per  liter. 
*  Annal.  chim.  anal,  appl.,  7,  165. 


ALUMINIUM,  BARIUM,  STRONTIUM,  CALCIUM.          209 


GROUP  V. 

This  group  includes  the  most  positive  metals  of  the  voltage 
series  which  cannot  be  deposited,  even  from  alkaline  solutions, 
except  in  the  form  of  amalgams.  With  the  exception  of  aluminium 
and  beryllium,  all  the  elements  of  this  group  are  the  so-called 
alkaline-earths  and  alkalies.  In  connection  with  this  group  the 
determination  of  various  anions  will  also  be  discussed. 

Aluminium. 

If  a  solution  of  aluminium-ammonium  oxalate,  containing  am- 
monium oxalate  in  excess,  is  submitted  to  the  action  of  the  electric 
current,  the  ammonium  oxalate  is  changed  into  carbonate  and  the 
aluminium  separates  as  hydroxide.  When  the  oxalate  is  decom- 
posed, the  solution  is  heated  until  there  is  only  a  faint  odor  of 
ammonia,  the  hydroxide  filtered  off,  washed  with  water  and  con- 
verted by  ignition  into  Al20s. 

This  behavior  of  aluminium  is  utilized  only  in  the  case  of  cer- 
tain separations. 

Barium,  Strontium,  Calcium. 

These  metals  can  be  deposited  electrolytically  from  their  aqueous 
solutions  only  in  the  form  of  amalgams.  Inasmuch  as  the  amal- 
gams of  the  light  metals  are  much  more  readily  decomposed  by 
water  than  are  the  amalgams  of  the  heavy  metals  (cf.  p.  206) 
they  cannot  be  determined  quantitatively  by  weighing  such  amal- 
gams. A.  Coehn  and  W.  Kettembeil  *  have  attempted  to  use  the 
amalgam  method  as  a  basis  for  separating  the  three  alkaline  earth 
metals  from  one  another  and  have  found  that  it  is  possible  to  effect 
such  a  separation  because  the  voltages  at  which  the  individual 
amalgams  are  formed  lie  far  enough  from  one  another  to  enable 
one  metal  to  be  deposited  completely  before  the  next  begins 
to  deposit.  Thus  the  potential  difference  between  barium  and 
strontium  amounts  to  0.2  volt,  between  strontium  and  calcium 
to  0.25  volt  and  between  barium  and  calcium  to  0.45  volt.  The 
determination  of  the  metal  after  it  has  been  deposited  in  the  mer- 

*  Z.  anorg.  Chem.,  38,  198  (1903). 


210  QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS 

cury  is  effected  by  titrating  the  hydroxide  which  is  formed  by 
the  action  of  water  upon  the  amalgam.  For  more  complete 
details,  see  the  chapter  on  The  Separation  of  the  Alkali  and 
Alkaline  Earth  Metals  from  Magnesium  and  from  the  Heavy 
Metals,  page  218. 

Determination  of  the  Halogens. 

The  method  originated  by  Vortmann  *  depends  upon  the  prin- 
ciple that  the  halogens  are  set  free  from  solutions  of  halogen  salts 
by  the  electric  current,  and  while  in  the  nascent  state  combine 
with  a  silver  anode  to  form  insoluble  silver  halide.  The  increase 
in  weight  of  the  anode  gives  directly  the  quantity  of  halogen  which 
has  separated.  The  completion  of  the  analysis  is  determined  by 
applying  one  of  the  usual  tests  for  halogen  or  by  replacing  the 
anode  with  the  silver  halide  upon  it  by  a  second  weighed  silver 
anode  and  seeing  whether  there  is  any  change  in  weight  with  the 
fresh  electrode. 

In  Vortmann's  method  the  following  apparatus  is  used  for  the 
determination  of  iodine.  The  anode  is  a  disk  of  pure  silver  having 
the  shape  of  a  6-cm.  watch  glass;  a  stout  platinum  wire  is  fastened 
to  the  center.  The  cathode  consists  of  a  copper  disk,  5  cm.  in 
diameter;  it  is  likewise  fastened  to  a  platinum  wire  and  has  a 
radial  section  cut  out  of  it  to  give  space  for  the  wire  of  the  silver 
anode,  which  is  placed  below  the  cathode.  It  is  advisable  to  insu- 
late the  wire  of  the  copper  cathode  by  means  of  rubber  tubing,  or 
by  sealing  it  in  glass  tubing.  As  electrolyzing  vessel,  a  crystalliz- 
ing dish  of  100  to  150  cc.  capacity  may  be  used;  during  the  elec- 
trolysis the  dish  is  covered  by  a  watch  glass  which  has  been  cut 
into  two  equal  pieces. 

Enough  of  the  iodide  to  correspond  to  about  0.1  to  0.25  gm.  of 
iodine  is  dissolved  in  water,  6  (or  10)  cc.  of  a  10-per-cent  caustic- 
soda  solution  is  added  and  the  alkaline  electrolyte  is  diluted  to 
100  (or  150)  cc.  The  silver  anode  is  placed  about  0.5  cm.  from  the 
bottom  of  the  dish  and  the  copper  cathode  about  2  cm.  above  the 
anode.  The  potential  of  the  current  should  lie  between  1.94  and 
2  volts  and  its  strength  should  be  from  0.03  to  0.07  ampere.  The 
electrolyte  is  not  heated.  As  soon  as  the  yellow  silver  iodide 
assumes  a  brownish-violet  tint  in  places,  the  solution  is  tested  by 

*  Monatsh.  Chem.,  16,  280  (1894);  16,  674  (1895). 


SEPARATION  OF  THE  HALOGENS   BY  ELECTRO-ANALYSIS  211 

taking  a  few  drops,  acidifying  with  sulphuric  acid,  and  adding 
potassium  nitrite;  an  iodide  will  give  free  iodine  in  this  test  and 
the  color  is  intensified  by  shaking  with  carbon  disulphide. 

When  the  electrolysis  is  finished,  the  current  must  be  stopped 
at  once  if  sulphate,  nitrate,  acetate  or  tartrate  is  present,  as 
otherwise  traces  of  silver  will  be  dissolved  from  the  anode  and 
carried  to  the  cathode.  The  silver  iodide  is  washed  with  water, 
dried  for  15  to  30  minutes  at  100°  to  110°  and  then  ignited.  For 
this  purpose  the  anode  is  placed  in  an  iron  dish  and  suspended 
about  0.5  cm.  from  the  bottom.  The  dish  is  covered  with  the 
two  halves  of  a  watch  glass  and  heated  until  the  silver  iodide  has 
assumed  a  bright  red  color,  or  until  it  begins  to  melt.  As  a  rule, 
black  specks  of  silver  peroxide  are  noticed  on  the  silver  iodide 
layer  and  these  result  at  places  where  gas  bubbles  prevented  the 
formation  of  silver  iodide  during  the  electrolysis.  If  this  is  the 
case,  the  heating  is  continued  only  until  the  black  points  have 
become  white.  The  electrode  is  then  cooled  in  a  desiccator  and 
weighed.  A  silver  anode  of  6  cm.  diameter  can  take  up  as  much 
as  0.5  gm;  of  iodine. 

If  less  than  0.02  gm.  of  iodine  is  to  be  determined  by  this  method, 
the  solution  of  the  iodide  is  treated  before  the  electrolysis  with 
only  3  cc.  of  caustic  soda  and  with  2  or  3  gms.  of  Rochelle  salt. 
The  purpose  of  the  latter  is  to  prevent  the  liquid  from  becoming 
turbid  by  some  of  the  silver-iodide  deposit  being  loosened  from  the 
anode. 

To  regenerate  the  silver  anode  after  the  analysis  is  finished,  it 
is  placed  as  cathode  in  a  platinum  dish  containing  dilute  caustic- 
soda  solution  and  the  dilute  alkali  is  electrolyzed  with  a  current  of 
2  volts.  A  light  layer  of  spongy  silver  is  formed  upon  the  disk 
and  is  easily  rubbed  off  (cf.  p.  214). 

Separation  of  the  Halogens  by  Electro-Analysis. 

By  using  the  silver  anodes  recommended  by  Vortmann,  and  by 
taking  advantage  of  the  principle  of  electrolytic  separation  by  the 
gradation  of  the  electromotive  force,  H.  Specketer  *  was  the  first 
to  succeed  in  separating  iodine,  bromine,  and  chlorine  from  one  an- 
other and  in  determining  the  first  two  of  these  elements  upon  the 
silver  anode.  Specketer  ascertained,  first  of  all,  the  decomposition 
potentials  for  potassium  iodide,  potassium  bromide  and  potassium 

*  Z.  anorg.  Chem.,  21,  273  (1899). 


212  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

chloride  in  a  normal  solution  of  sulphuric  acid  and  found  that  if, 
during  the  electrolysis  of  a  mixture  of  the  three  halogen  salts,  the 
potential  was  not  allowed  to  rise  above  0.13  volt,  the  iodine  was 
deposited  with  sufficient  accuracy  upon  the  silver  anode  without 
any  admixture  of  bromine  or  chlorine. 

To  separate  the  bromine  from  chlorine,  a  potential  of  0.35  volt 
must  be  employed;  if  this  voltage  is  not  exceeded,  the  bromine 
deposits  upon  the  silver  anode  free  from  chlorine. 

The  electrolytic  determination  of  chlorine  in  this  way  offers 
considerable  difficulty  and  the  method  certainly  has  no  advantage 
over  the  volumetric  method  for  determining  chlorine. 

The  principal  conditions,  then,  for  carrying  out  a  satisfactory 
separation  of  the  halogens  are,  first,  to  maintain  a  definite  degree 
of  acidity  in  the  electrolyte;  second,  a  constant  potential  of  the 
bath;  and  third,  to  keep  the  solution  out  of  contact  with  oxygen. 
This  last  requirement,  the  necessity  for  which  is  explained  a  little 
later  on,  is  satisfied  by  passing  hydrogen  gas  through  the  solution. 
Apparatus.  The  source  of  current  used  by  Specketer  was  a 

Gtilcher  thermopile  which  was 
short  circuited  with  a  resistance 
wire  bearing  a  sliding  contact 
(cf.  p.  132).  If  storage  cells  are 
used,  this  wire  is  connected  across 
the  binding  posts  A B,  at  which 
/T~L\_J  the  current  is  usually  taken  for 

\LJ/  the  electrolysis.     The  current  is 

FIQ  5Q  "   now  taken  from  two  points  A  and 

6  between  which  the  voltmeter  V 

shows  the  desired  potential  difference.  A  sensitive  ammeter, 
Amp.,  is  used  for  measuring  the  strength  of  the  current. 

As  electrolyzing  vessel  a  narrow  cylinder  is  used  which  is  tall 
enough  to  prevent  losses  by  spattering  when  hydrogen  is  passed 
through  the  solution.  The  hydrogen  is  taken  from  a  Kipp  gen- 
erator and  enters  the  solution  near  the  bottom  of  the  cylinder, 
through  a  glass  tube  drawn  out  to  a  capillary  at  the  end.  The 
top  of  the  cylinder  contains  a  cork  stopper  with  perforations 
through  which  the  gas  delivery  tube  passes,  as  well  as  the  electrode 
wires;  the  cork  fits  loosely  in  the  cylinder  to  permit  the  escape 
of  hydrogen  gas.  As  cathode,  platinum  foil  is  used,  and  as  anode 
a  piece  of  gauze  made  from  thin  silver  wire.  It  is  necessary  to 


SEPARATION  OF  BROMINE   FROM   CHLORINE  213 

use  pure  silver  because  impurities,  such  as  copper,  will  be  dissolved 
during  the  electrolysis  and  pass  to  the  anode. 

Separation  of  Iodine  from  Bromine  and  Chlorine. 

The  halogen  salts  are  dissolved  in  100  cc.  of  normal  sulphuric 
acid  and  electrolyzed,  while  passing  hydrogen  through  the  cell, 
at  a  potential  of  0.13  volt,  until  there  is  no  further  evolution  of 
hydrogen  at  the  cathode  and  the  ammeter  no  longer  shows  any 
deflection.  If  a  sensitive  ammeter  is  not  at  hand,  the  end  of  the 
electrolysis  is  determined  by  testing  for  iodine  by  means  of  bromine 
water  and  starch.  Toward  the  end  of  the  process  it  is  necessary 
to  wash  down  the  sides  of  the  cylinder. 

When  all  the  iodine  has  been  deposited,  the  current  is  turned 
off,  the  anode  rinsed  in  the  usual  way  and  dried  at  120°.  If, 
besides  the  iodine,  only  one  other  halogen  is  present,  it  is  simplest 
to  determine  the  bromine  or  chlorine  by  titration.  If,  however, 
both  bromine  and  chlorine  are  present,  the  bromine  is  also  deposited 
by  the  current. 

Separation  of  Bromine  from  Chlorine. 

The  electrolysis  is  carried  out  in  exactly  the  same  way  as  before 
except  that  now  the  current  is  kept  at  0.35  volt.  Since  the  solu- 
tion has  been  diluted  by  washing  the  anode  during  the  preceding 
analysis,  it  is  necessary  to  restore  the  acidity  to  that  of  a  normal 
solution  by  the  addition  of  stronger  sulphuric  acid  of  known  acid 
strength.  As  the  test  for  traces  of  bromine  in  the  presence  of 
chloride  by  means  of  chlorine  water  and  carbon  disulphide  is  not 
very  delicate,  it  is  better  to  determine  the  end  of  the  electrolysis 
with  the  aid  of  a  sensitive  galvanometer. 

After  the  removal  of  the  bromine,  the  chlorine  remaining  in 
solution  is  always  determined  by  titration  because  it  is  impossible 
to  determine  chlorine  upon  a  silver  anode  without  some  silver 
passing  into  solution. 

Instead  of  treating  the  silver  anode  as  described  on  page  211, 
to  free  it  from  halogen,  it  may  be  reduced  by  using  it  as  cathode 
in  the  electrolysis  of  an  approximately  normal  solution  of  sulphuric 
acid ;  another  simple  method  is  the  use  of  zinc  and  dilute  sulphuric 
acid. 

The  principal  points  to  be  observed  in  the  above-described 
method  for  separating  the  halogens  are:  (1)  a  constant  current  with 


::~       QCAXTTTATFTE  ANALYSE  BY  ELECTBOLYSBS 


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erf  1.08  Toils  k 


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(pu  205).  J.  H.  Hfldebrand 
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I.  F. 


ELECTROLYTIC  DETERMINATION  OF  HALOGENS      215 

way  that  the  halogen  is  caused  to  unite  with  the  silver  while  the 
metal  forms  an  amalgam  with  the  mercury;  on  decomposing  the 
amalgam  with  water,  a  solution  of  alkali  hydroxide  is  obtained  and 
can  be  titrated  against  a  standardized  solution  of  acid. 

If  such  an  analysis  is  carried  out  with  the  silver  anode  and  ite 
halogen  deposit  in  the  same  electrolysing  dish  that  contains  the 
alkali  amalgam,  then,  even  during  the  electrolysis,  some  alkali 
hydroxide  is  formed  as  a  result  of  the  action  of  water  upon  the 
amalgam,  and  when  the  current,  after  the  halogen  salt  has  been 
completely  decomposed,  acts  upon  this  solution  of  alkali  hydroxide, 
some  silver  oxide  is  formed  at  the  anode  whereby  its  weight  is 
gradually  increased.  It  is,  therefore,  necessary  to  know  just  when 
the  last  of  the  halogen  is  deposited  and  to  stop  the  electrolysis 
tbea, 

To  overcome  this  difficulty,  Hildebrand  made  use  of  the  prin- 
ciple which  is  involved  in  the  technical  preparation  of  caustic 
soda;  the  alkali  halide  is  electrolysed  and  the  amalgam  decomposed 
in  different  compartments.  The  electrolyzing  vessel  (Fig.  51) 
consists  of  a  crystallizing  dish  11  cm.  hi  diameter  and  5  cm.  deep 
in  which  a  glass  cylinder  4.5  cm.  high  rests  upon  a  thin  glass  rod 


PIG.  51. 


bent  into  a  triangle.    The  glass  cylinder  is  obtained  by  cutting 
off  the  bottom  of  a  beaker.     It  is  held  firmly  in  place,  by  wedg- 


216  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

ing  three  corks  between  it  and  the  sides  of  "the  crystallizing  dish, 
so  that  it  does  not  float  when  mercury  is  added.  Enough  mer- 
cury is  poured  into  the  dish  to  make  its  surface  about  3  mm. 
above  the  bottom  of  the  inner  cylinder  and  thus  liquid  poured 
upon  the  mercury  is  kept  perfectly  isolated  in  the  two  compart- 
ments. The  inner  space  is  for  the  salt  solution,  into  which  the 
silver  anode  dips.  The  end  of  a  short  platinum  wire  dips  into 
the  mercury  near  the  walls  of  the  dish  and  it  is  connected  with  the 
negative  pole  in  the  following  manner:  a  piece  of  platinum  wire 
is  melted  into  the  bottom  of  a  narrow  glass  tube  so  that  it  extends 
about  1  cm.  within  and  without  the  tube;  mercury  is  poured  into 
the  tube  and  into  this  dips  a  copper  wire  leading  to  the  negative 
binding  post.  All  the  mercury  in  the  bottom  of  the  dish  serves 
as  the  cathode. 

A  little  water  and  a  few  cubic  centimeters  of  a  saturated  salt 
solution  are  poured  upon  the  mercury  in  the  outer  compartment. 
When  the  current  is  turned  on,  the  silver  anode  in  the  middle  takes 
up  the  halogen  from  the  solution  in  the  inner  space  while  the 
alkali  metal  combines  with  the  mercury  at  the  bottom  to  form  an 
amalgam.  This  amalgam  distributes  itself  throughout  the  entire 
mercury  and  when  it  comes  in  contact  with  the  water  in  the  outer 
compartment,  it  is  decomposed  with  the  formation  of  alkali 
hydroxide.  Thus  the  mercury  on  the  outside  becomes  impover- 
ished of  its  amalgam  and,  as  a  result,  that  present  in  the  middle 
of  the  dish  constantly  diffuses  toward  the  outside.  When  all  of 
the  halogen  salt  within  the  inner  compartment  has  been  decom- 
posed, all  of  the  hydroxide  will  be  found  in  the  outer  space  and 
only  pure  water  will  surround  the  anode.  To  hasten  the  decom- 
position of  the  amalgam,  a  nickel  wire  bent  into  a  ring  is  placed 
inside  the  dish  about  1  cm.  above  the  surface  of  the  mercury;  the 
wire  rests  upon  four  wire  supports  as  shown  in  the  drawing.  If 
enough  dilute  salt  solution  has  been  poured  upon  the  mercury  to 
cover  this  wire,  a  galvanic  element  is  now  obtained,  consisting  of 
alkali  metal  —  salt-solution  —  nickel,  and  this  serves  to  hasten  the 
decomposition  of  the  amalgam  so  that  at  the  end  of  the  electrolysis 
only  pure  water  remains  in  the  inner  space.  The  salt  is  added 
merely  to  lessen  the  resistance  in  this  galvanic  cell. 

The  anode  recommended  by  Hildebrand  consists  of  two  disks 
of  platinum  gauze  having  about  300  meshes  to  the  square  centi- 
meter. The  disks  are  about  5  cm.  in  diameter  and  the  ends  of 


ELECTROLYTIC   DETERMINATION   OF   HALOGENS       217 

the  wire  are  fused  together  at  the  periphery,  by  means  of  the 
blast,  in  order  to  make  the  disks  a  little  stouter.  The  two  disks, 
each  weighing  about  16  gms.  and  each  having  about  75  sq.  cm.  of 
surface,  are  fastened  about  5  mm.  apart  upon  a  platinum  wire 
which  is  about  1  mm.  thick  and  10  cm.  long;  the  upper  end  of  the 
wire  is  bent  into  a  loop  so  that  it  may  be  suspended  from  the  beam 
of  the  balance. 

The  platinum  disks  are  plated  with  from  3  to  4  gms.  of  silver 
which  is  deposited  upon  them  from  a  cyanide  solution  (cf.  p.  133), 
with  a  current  of  1  or  2  amperes  and  a  rotating  anode.  With 
this  quantity  of  silver  many  halogen  determinations  may  be 
made,  dissolving  the  silver  halide  after  each  analysis  by  letting 
the  electrode  remain  in  potassium-cyanide  solution  for  a  short 
time. 

Procedure.  There  are  several  reasons  why  it  is  advisable  to 
keep  the  anode  revolving:  the  silver  compounds  adhere  better 
to  the  electrode  and  the  motion  of  the  electrolyte  is  imparted  to  the 
mercury  at  the  bottom  of  the  dish  so  that  the  amalgam  in  the 
middle  of  the  dish  is  more  quickly  transported  to  the  outer  space. 
The  stirring  has  less  result  upon  the  duration  of  the  analysis,  for 
as  the  electrolysis  proceeds  the  composition  of  the  electrolyte 
approaches  that  of  pure  water;  the  resistance  then  increases  and 
the  current  sinks  so  that  it  requires  a  long  time  for  the  decomposi- 
tion of  the  last  traces  of  the  salt. 

The  solution  of  about  0.1  gm.  of  the  salt  to  be  analyzed  is  poured 
into  the  inner  compartment  and  the  anode  is  fixed  in  place  so  that 
the  lower  disk  is  about  5  mm.  above  the  surface  of  the  mercury; 
it  is  made  to  revolve  at  a  rate  of  250  to  300  times  per  minute.  If 
necessary,  enough  water  is  added  so  that  the  upper  disk  is  covered 
by  liquid.  If  only  little  halogen  salt  is  present,  however,  it  suffices 
in  most  cases  to  keep  only  the  lower  disk  covered  with  solution. 

For  the  analysis  of  potassium  chloride  and  of  potassium  bromide, 
Hildebrand  started  with  a  current  of  5  volts  and  0.65  ampere; 
the  electrolysis  was  continued  until  the  strength  of  the  current  fell 
to  0.01  ampere  and  this  required  30  minutes  with  0.1  gm.  potassium 
bromide. 

Potassium  ferrocyanide  was  electrolyzed  with  an  initial  voltage 
of  3  to  4  volts  and  an  initial  current  of  0.15  to  0.2  ampere.  Potas- 
sium ferricyanide  gave  good  results  with  a  current  of  0.2  to  0.4 
ampere  at  2  to  4.5  volts. 


218  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

In  the  analysis  of  sodium  carbonate  it  is  best  to  have  both 
disks  covered  with  electrolyte  and  a  somewhat  roughened  silver 
surface  is  desirable.  The  roughening  is  accomplished,  after  the 
electrode  has  been  plated  in  a  cyanide  solution  with  the  anode 
rotating,  by  allowing  the  electrolysis  to  continue  for  a  short  time 
with  a  stationary  electrolyte.  The  analysis  of  the  carbonate  is 
carried  out  slowly,  with  an  initial  potential  of  3.5  to  4  volts  and 
a  current  of  only  0.15  ampere.  The  analysis  requires  from  60  to 
90  minutes.  After  weighing  the  deposit  of  silver  carbonate,  the 
best  way  to  prepare  the  electrode  for  future  use  is  to  ignite  it 
gently. 

In  the  analysis  of  normal  sodium  phosphate,  the  potential  of  the 
current  must  not  exceed  4  volts,  if  a  good  deposit  is  desired.  The 
current  should  not  be  more  than  0.3  to  0.4  ampere  at  the  start. 

Owing  to  the  poor  conductivity  of  this  salt  and  the  low  voltage 
employed  in  the  analysis,  the  time  required  is  75  to  120  minutes. 
The  duration  of  the  experiment  may  be  shortened  to  about  an 
hour  if  a  fresh  electrode  is  used  for  the  deposition  of  the  last 
traces  of  phosphoric  acid. 

Since,  in  all  these  cases,  the  solution  in  the  interior  compart- 
ment is  nearly  pure  water  at  the  last,  it  is  hardly  necessary  to 
wash  the  electrode  with  water;  rinsing  with  alcohol  and  then  with 
ether  should  suffice. 

When  the  decomposition  of  the  solution  is  complete,  the  entire 
contents  of  the  crystallizing  dish  are  transferred  to  a  beaker,  the 
dish  rinsed,  and  the  alkaline  solution  titrated  with  tenth-normal 
sulphuric  acid,  using  methyl  orange  as  indicator. 

McCutcheon  and  Lukens,  in  studying  the  determination  of 
halogens  by  means  of  the  Hildebrand  decomposition  vessel,  devised 
the  following  method  of  separation.* 

Separation  of  Alkali  and  Alkaline    Earth    Metals    from    the 
Heavy  Metals. 

In  general,  it  has  been  found  that  when  metal  chlorides  are 
decomposed  in  Hildebrand' s  apparatus,  the  amalgams  of  lithium, 
sodium,  potassium,  calcium,  strontium  and  barium  diffuse  into 
the  outer  compartment  and  there,  after  being  decomposed  by  the 
water,  the  resulting  hydroxides  may  be  determined  by  titration; 

*  The  separations  to  be  described  really  belong  in  Part  III  but  will  be 
understood  more  readily  if  described  here. 


SEPARATION  OF  ALKALI  AND  ALKALINE  EARTHS     219 

whereas  the  amalgams  of  magnesium,  cadmium,  tin,  antimony, 
iron,  aluminium,  chromium,  manganese,  zinc,  nickel,  cobalt, 
titanium,  uranium,  vanadium,  zirconium,  thorium,  lanthanum, 
cerium,  neodymium  and  praseodymium  remain  in  the  inner  com- 
partment and  are  decomposed  by  water  forming  insoluble  hydrox- 
ides. By  this  classification  the  general  way  of  separating  metals 
of  the  two  groups  is  indicated.  The  behavior  of  calcium  when 
magnesium  is  also  present  is,  however,  quite  different  from  that 
of  the  other  members  of  the  group.  In  this  case  the  calcium 
amalgam  behaves  as  if  it  belonged  to  the  second  group,  i.e.,  it  is 
deposited  together  with  the  magnesium  in  the  middle  compart- 
ment as  insoluble  hydroxide. 

Although  this  makes  it  impossible  to  separate  calcium  and 
magnesium  under  the  ordinary  conditions  of  the  analysis,  yet 
advantage  can  be  taken  of  this  behavior  when  it  is  desired  to 
separate  metals  such  as  potassium,  sodium,  lithium  and  especially 
barium  and  strontium  from  calcium;  it  is  only  necessary  to  add 
magnesium  chloride,  in  case  it  is  not  already  present. 

To  illustrate  the  usefulness  of  the  method  the  separations  de- 
scribed by  H.  S.  Lukens  and  E.  F.  Smith  will  be  given. 

Barium  chloride,  in  solutions  containing  about  0.228  gm.  of 
Ba,  was  decomposed  quantitatively  in  50  to  60  minutes,  by  a 
current  of  0.3  ampere  at  3.5  to  4  volts.  The  anode  made  300 
revolutions  per  minute  and  contained  all  the  chlorine  in  the  form 
of  chloride. 

The  details  of  the  metal  determination  will  be  given  later. 

Strontium  bromide,  in  a  solution  containing  0.727  gm.  of  Sr, 
was  electrolyzed  under  the  same  conditions  with  the  same  success. 
In  this  particular  case  the  bromine  was  not  determined. 

Separation  of  Potassium  or  Sodium  from  Calcium  and  Magne- 
sium. Solutions  containing  0.022  gm.  Ca,  0.0210  gm.  Mg  as 
chloride,  in  one  experiment  with  0.0474  gm.  Na  and  in  another 
with  0.0582  gm.  K,  likewise  as  chloride,  were  electrolyzed  with  a 
current  of  0.25  ampere  at  3.5  volts  in  50  minutes;  all  the  sodium, 
or  potassium,  was  obtained  by  titrating  the  solution  in  the  outer 
compartment.  In  these  separations,  no  weight  is  placed  upon  the 
determination  of  the  halogen. 

Barium  or  Strontium  from  Calcium  and  Magnesium.  The  solu- 
tion contained  in  one  case  0.0222  g.  Ca,  0.021  gm.  Mg  and  0.0455 
Ba  as  chloride;  in  another  case  twice  as  much  Ba,  0.091  gm.; 


220  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

in  a  third  case  0.0563  gm.  of  Sr  as  bromide  instead  of  the  barium 
chloride.  The  current  used  was  0.3  ampere  at  3.5  to  4  volts. 
To  effect  the  deposition  of  the  last  traces  of  barium  or  strontium, 
it  was  found  necessary  toward  the  last  to  add  a  few  drops  of  hydro- 
chloric acid  to  the  solution.  Calcium  and  magnesium  were  found 
in  the  inner  compartment  in  the  form  of  an  insoluble  hydroxide 
precipitate  which  could  be  titrated  (see  below). 

Barium  or  Strontium  from  Magnesium  alone.  The  solutions 
contained  0.0358  gm.  Mg  and  either  0.0455  gm.  Ba  or  0.0221  gm. 
Sr.  The  conditions  were  the  same  as  when  calcium  was  present 
with  the  magnesium. 

Metals  of  the  Alkali  or  Alkaline-earth  Group  from  Iron.  The 
metals  were  present  as  chloride,  in  quantities  designated  below, 
and  the  current  was  3  to  5  volts  and  0.3  ampere.  The  experiments 
required  not  more  than  50  minutes  in  any  case.  The  iron  began  to 
deposit  as  hydroxide  almost  immediately: 

Barium  from  Iron:  0.0455  gm.  Ba,  0.0276  gm.  Fe? 

Strontium  from  Iron:  0.0565  gm.  Sr,  0.0276  gm.  Fe. 

Potassium  from  Iron:  0.0580  gm.  K,  0.0276  gm.  Fe. 

Sodium  from  Iron:  0.0474  gm.  Na,  0.0276  gm.  Fe. 

Metals  of  the  Alkali  and  Alkaline-earth  Groups  from  Aluminium. 
The  solutions  contained  the  metals  as  chloride  and  the  conditions 
were  the  same  as  before: 

Barium  from  Aluminium:  0.0455  gm.  Ba,  0.0199  gm.  Al. 

Strontium  from  Aluminium:  0.0221  gm.  Sr,  0.0199  gm.  Al. 

Potassium  from  Aluminium:  0.0580  gm.  K,  0.0199  gm.  Al. 

Sodium  from  Aluminium:  0.0474  gm.  Na,  0.0199  gm.  Al. 

In  all  the  above  cases  only  the  metal  obtained  as  hydroxide 
in  the  outer  compartment  was  determined  by  titration.  For 
the  determination  of  the  calcium,  whose  hydroxide  is  the  least 
soluble  and  whose  amalgam  is  the  most  difficult  to  decompose, 
T.  P.  McCutcheon  proceeded  as  follows: 

The  liquid  in  the  interior  compartment  was  removed,  with  the 
aid  of  a  pipette  or  siphon,  and  the  inner  walls  as  well  as  the  mer- 
cury surface  were  thoroughly  washed  with  water.  Then  the  entire 
contents  of  the  crystallizing  dish  were  transferred  to  a  wide  beaker 
and  the  mercury  was  thoroughly  stirred  with  a  glass  rod,  the  end 
of  which  was  covered  with  a  piece  of  rubber  tubing;  this  stir- 
ring served  to  hasten  the  complete  decomposition  of  the  calcium 


POTASSIUM,  AMMONIUM   (NITROGEN)  221 

amalgam.  An  excess  of  tenth-normal  acid  was  added  and  the 
excess  finally  titrated  with  tenth-normal  alkali  solution,  using 
methyl  orange  as  indicator. 

The  same  author  accomplished  a  separation  of  calcium  from  mag- 
nesium by  using  a  current  of  higher  voltage.  A  solution  contain- 
ing 0.1  gm.  MgCl2  and  0.0771  gm.  CaCl2  was  electrolyzed  with  an 
initial  current  of  0.3  ampere  at  9  volts;  the  current  dropped  to  0.2. 
ampere  toward  the  last.  Decomposition  was  complete  in  3  hours* 
so  that  the  calcium  could  be  determined  in  the  outer  compart- 
ment by  the  method  given  above.  A  few  drops  of  hydrochloric 
acid  were  added  from  time  to  time  to  the  inner  solution,  in  order 
to  hasten  the  electrolysis. 

McCutcheon  also  carried  out  the  following  separations: 

Sodium  from  Uranium.  Used  0.1  gm.^of  uranyl  chloride  and 
0.1172  gm.  NaCl;  3  volts,  0.3  ampere;  time,  3  hours.  The  cur- 
rent at  the  end  had  fallen  to  0.02  ampere  and  the  potential  had 
increased  to  5  volts. 

Potassium  from  Uranium.  Used  0.1  gm.  U02C12,  0.1467  gm. 
KC1;  initial  potential  3  volts,  final  potential  5  volts;  0.5  ampere 
at  the  start  and  0.01  ampere  at  the  last;  time,  2  hours. 

Lithium  from  Uranium.  Used  0.1  gm.  UO2C12,  0.0846  gm.  LiCl; 
potential  5  volts;  current  strength  0.30  to  0.02  ampere;  time, 
2  hours. 

Barium  from  Uranium.  Used  0.1  gm.  U02C12,  0.104  gm. 
BaCl2;  potential  5  volts,  current  strength  0.15  to  0.01  ampere. 
A  little  hydrochloric  acid  was  added,  as  mentioned  above;  tune, 
1  hour. 

Strontium  from  Uranium.  Used  0.1  gm.  UO2C12,  0.1456  gm. 
SrBr2;  potential  5  volts,  current  strength  0.4  to  0.02  ampere; 
time,  2  hours. 

Under  similar  conditions,  separations  of  barium  from  thorium, 
cerium,  lanthanum,  and  neodymium  were  carried  out  successfully. 

Potassium,  Ammonium  (Nitrogen). 

The  usual  gravimetric  method  for  determining  potassium  and 
ammonium  is  to  form  the  potassium  or  ammonium  chloroplati- 
nate  and  weigh  it  upon  a  tared  filter,  or  in  a  Gooch  crucible,  after 
drying  at  110°.  The  weighing  of  the  filter  after  such  drying  is 
always  more  or  less  inaccurate  and  it  is  better  to  dissolve  the 
chloroplatinate  in  hot  water,  determine  the  platinum  electrolyti- 


222  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

cally  and  from  this  compute  the  potassium  or  ammonium  con- 
tent.    The  electrolysis  is  carried  out  as  described  on  page  140. 

O.  Schumm  *  has  used  this  method  to  advantage  in  the  deter- 
mination of  the  potassium  in  blood. 

Determination  of  Nitric  Acid  in  Nitrates. 

In  connection  with  Luckow's  observation  that  a  sulphuric- 
acid  solution  of  pure  alkali  nitrate,  on  being  electrolyzed 
with  platinum  electrodes,  is  not  transformed  completely  into 
ammonia  although  a  quantitative  transformation  takes  place  if 
copper  sulphate  is  also  present  in  the  solution,  G.  Vortmann  | 
was  led  to  determine  the  conditions  under  which  the  quantitative 
determination  of  nitric  acid  in  nitrates  could  be  made.  Under 
these  conditions,  the  copper  is  deposited  upon  the  cathode  and 
the  nitric  acid  is  reduced  to  ammonia  so  that  eventually  all  the 
nitrogen  is  present  as  ammonium  sulphate,  provided  a  sufficient 
quantity  of  free  sulphuric  acid  was  present  at  the  start.  The 
liquid  is  subjected  to  distillation  after  adding  a  large  excess  of 
caustic  soda  and  the  distillate  is  caught  in  a  measured  volume  of 
standardized  sulphuric-acid  solution.  The  excess  of  the  latter  is 
finally  determined  by  titration  with  caustic-soda  solution,  and 
from  this  the  quantity  of  nitric  acid  is  computed. 

In  the  course  of  time,  the  method  has  experienced  some  modi- 
fication. K.  Ulsch  J  found  that  the  reduction  of  the  nitric  acid 
to  ammonia  took  place  if  a  spiral  of  copper  wire  was  used  as  the 
cathode,  which  was  ignited  just  before  starting  the  analysis  and 
plunged  into  cold  water.  The  addition  of  copper  sulphate  to  the 
electrolyte  is  then  unnecessary.  Moreover,  Ulsch  found  that  if 
a  measured  quantity  of  standardized  sulphuric  acid  was  added  to 
the  solution  of  the  nitrate,  then  after  the  electrolysis  the  acidity 
of  the  solution  could  be  determined  by  titration  and  thus  the 
quantity  of  ammonia  formed  could  be  at  once  computed  and  the 
distillation  avoided.  It  is  necessary  to  bear  in  mind  that  only 
half  of  the  sulphuric  acid  neutralized  during  the  experiment 
is  combined  with  ammonium  for  the  other  half  is  combined 
with  potassium  (in  case  potassium  nitrate  was  analyzed).  It  is 
also  assumed  that  the  solution  of  the  nitrate  was  neutral  at  the 

*  Z.  anal.  Chem.,  40,  385  (1901). 

t  Ber.,  23,  2798  (1890). 

j  Z.  Elektrochem.,  3,  546  (1897). 


DETERMINATION  OF  NITRIC  ACID  IN   NITRATES      223 

start.  Such  a  simplification  of  the  process  was  suggested  by  Vort- 
mann  and  later  used  by  L.  H.  Ingham.*  The  last-named  author 
shortened  the  operation  by  stirring  the  electrolyte.  W.  H.  Easton 
sought  to  determine  the  most  favorable  experimental  conditions 
with  stationary  electrolytes. 

Up  to  the  present  time,  however,  the  method  has  nearly  always 
been  tested  with  pure  potassium  nitrate;  further  experiments  are 
desirable  to  see  whether  the  method  is  applicable  to  all  sorts  of 
products  such  as  those  which  have  to  be  analyzed  in  agricultural 
experiments  stations.  Above  all  it  would  be  desirable  to  know 
what  effect  the  usual  impurities,  such  as  chloride,  etc.,  have  upon 
the  results.  On  account  of  the  uncertainty  which  is  still  attached 
to  the  method  there  will  be  given  here  only  a  statement  of  the 
working  conditions  which  Easton  found  to  be  the  most  favorable. 

From  0.1  to  0.5  gm.  of  potassium  nitrate  and  an  equal  quantity 
of  copper  sulphate  are  dissolved  in  water,  30  cc.  of  sulphuric  acid 
(sp.  gr.  1.062)  are  added,  and,  after  diluting  to  150  cc.,  the  elec- 
trolysis is  carried  out  with  a  platinum  cathode  using  a  current  of 
NDioo  =  1  ampere  at  the  ordinary  temperature.  After  2.5  hours 
all  the  copper  will  have  been  deposited  on  the  cathode,  in  an  ad- 
herent form,  and  all  the  nitric  acid  transformed  into  ammonium 
sulphate.  The  solution  is  poured  out  of  the  beaker,  evaporated 
with  the  washings,  and  distilled  with  an  excess  of  caustic  soda. 
The  excess  of  sulphuric  acid  in  the  receiver  is  determined  by 
titration  with  caustic  alkali,  using  methyl  orange  as  indicator,  t 

*  J.  Am.  Chem.  Soc.,  26,  1251  (1904). 

t  In  the  determination  of  the  nitrogen  in  organic  substances,  G.  Budde  and 
C.  Schou  (Z.  anal.  Chem.,  38,  344  (1899))  have  attempted  to  decompose  the 
substance  in  concentrated  sulphuric  acid  with  the  aid  of  the  electric  current, 
without  adding  any  of  the  ordinary  substances  recommended  for  the  Kjeldahl 
method.  This  process,  however,  has  not  proved  universally  applicable. 


224  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 


Preparation  of  Standard  Sulphuric  Acid  Solution. 

When  a  neutral  solution  of  copper  sulphate  is  electrolyzed,  the 
changes  that  take  place  may  be  represented  by  the  following 
equations : 

cathode,     2Cu  +  +  +  4  ©  -»  2Cu; 
anode,     2H20  +  4  0  ->  4H+  +  02  T  . 

After  all  the  copper  is  deposited,  hydrogen  ions  are  discharged 
at  the  cathode  just  as  fast  as  they  are  formed  at  the  anode. 

4H+  +  4  0  -»2H2  T         2H20  +  40^  4H+  +  02  T  • 

The  acidity  of  the  solution,  therefore,  can  be  computed  either 
from  the  weight  of  copper  sulphate  decomposed  or  from  the 
weight  of  copper  deposited.  From  1.249  gm.  of  CuS04-5H2O 
or  for  0.3179  gm.  of  deposited  copper,  acid  equivalent  to  100  cc. 
of  tenth-normal  solution  is  obtained.  The  solution  may  be  used 
for  accurately  standardizing  a  solution  of  alkali. 

In  carrying  out  the  electrolysis  a  spongy  deposit  of  copper 
is  likely  to  be  obtained  with  a  stationary  electrolyte  if  the  cur- 
rent density  is  over  NDioo  =  0.4  ampere.  More  current  may 
be  used  if  the  anode  is  rotated,  the  solution  is  kept  warm,  and 
a  gauze  cathode  is  used. 


PART  III. 

SEPARATION   OF   METALS. 
COPPER. 

Separation  of  Copper  from  Silver. 

In  Nitric-acid  Solution.  The  separation  depends  upon  the  fact 
that  silver  can  be  deposited  at  a  certain  low  voltage  at  which 
copper  is  not  deposited.  It  is  of  great  importance  to  keep  the 
voltage  within  certain  limits  throughout  the  electrolysis.  The 
method  of  Ktister  and  v.  Stein wehr  was  described  on  page  131. 

If  the  copper  solution  has  become  too  dilute  by  the  washing  of 
the  silver  deposit,  it  is  concentrated  by  evaporation  and  the  copper 
is  deposited  in  the  nitric  acid  solution  as  described  on  page  124. 

In  Potassium-cyanide  Solution,  according  to  E.  F.  Smith  and 
L.  K..Frankel.*  When  the  solution  contains  0.1  to  0.2  gm.  of  silver 
and  about  0.2  gm.  copper,  it  suffices  to  add  2  gms.  of  pure  potas- 
sium cyanide;  if  more  copper  is  present,  e.g.,  0.5  gm.,  twice  as 
much  potassium  cyanide  is  added.  The  solution  is  diluted  to 
about  125  cc.  and  electrolyzed  at  65°  to  75°  with  a  current  density 
of  NDioo  =  0.03  to  0.07  ampere  at  1  to  1.4  volts.  According  to 
the  quantity  of  silver,  the  separation  requires  from  4  to  8  hours. 
The  determination  of  the  copper  will  be  discussed  on  page  226. 

The  above  two  separations  are  based  upon  two  different  prin- 
ciples. In  the  first  method,  the  silver  is  deposited  at  a  voltage  so 
low  that  no  copper  is  deposited  from  a  nitric-acid  solution;  in  the 
second  method,  both  metals  are  converted  into  complex  salts  of 
which  one  is  more  stable  than  the  other.  In  the  first  case  it  is 
necessary  to  keep  the  voltage  within  certain  limits,  if  the  copper 
is  to  remain  in  solution ;  and  in  the  second  case  it  is  necessary  that 
the  cuprocyanide  ion  shall  retain  its  strongly  complex  character 
until  all  the  silver  is  deposited,  or,  in  other  words,  the  secondary 
dissociation  of  the  complex  cuprocyanide  ion  must  be  prevented 
as  far  as  possible.  Strictly  speaking,  both  methods  depend  upon 

*  E.  F.  Smith,  Electrochemical  Analysis. 
225 


226          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

the  fact  that  different  electromotive  forces  are  necessary  to  deposit 
the  two  elements,  for  this,  as  was  stated  on  page  80,  is  the  general 
principle  upon  which  all  electrolytic  separations  are  based. 

From  what  has  been  said  above,  as  well  as  on  page  51,  the 
.necessity  of  a  large  excess  of  potassium  cyanide  to  form  the 
cuprocyanide  complex  is  apparent.  Since  potassium  cyanide  is 
itself  decomposed  by  the  action  of  the  electric  current,  there  is 
another  reason  for  adding  an  excess  of  this  salt.  0.  Brunck  * 
has  shown  that  it  is  sufficient  to  add  for  100  cc.  of  liquid  2  gms. 
of  potassium  cyanide  more  than  the  quantity  necessary  to  form 
the  complex  salt.  Under  these  conditions  it  is  possible  to 
separate  even  small  quantities  of  silver  from  large  quantities  of 
copper,  because  it  is  possible  to  use  voltages  far  above  the  decom- 
position potential  of  copper  from  normal  cupric  ions.  Thus 
Brunck  obtained  very  accurate  silver  determinations  in  2  or  3 
hours  under  the  following  conditions  of  working.  The  nitric-acid 
solution,  which  may  contain  from  0.24  to  0.05  gm.  of  stiver  and 
from  0.08  to  0.43  gm.  of  copper,  was  neutralized  with  caustic- 
potash  solution;  3  or  4  gms.  of  potassium  cyanide  and  0.5  gm. 
of  solid  potassium  hydroxide  were  added,  and  after  diluting  to 
100  cc.  the  solution  was  electrolyzed  at  the  laboratory  temper- 
ature using  a  platinum  gauze  electrode  and  a  current  at  2.5  to 
4  volts.  The  current  density  under  these  conditions  was  NDioo  = 
0.45  to  0.25  ampere. 

If  the  quantity  of  copper  is  large  in  proportion  to  the  quantity 
of  silver,  it  is  advisable  to  keep  the  current  density  down  to  0.25 
ampere  or  to  use  a  correspondingly  larger  quantity  of  potassium 
cyanide,  to  prevent  the  dissociation  of  the  complex  cuprocyanide 
ion.  The  small  quantity  of  solid  potassium  hydroxide  is  added 
to  prevent  the  formation  of  paracyanogen  which  separates  out  at 
the  anode  when  stronger  currents  are  employed;  the  cyanogen,  as 
fast  as  it  is  set  free  by  the  action  of  the  current,  unites  with  potas- 
sium hydroxide  to  form  potassium  isocyanate.  If,  toward  the 
end  of  the  electrolysis,  a  little  copper  should  deposit,  on  account  of 
the  presence  of  insufficient  potassium  cyanide,  this  is  shown  by 
the  reddish  tint  which  the  silver  deposit  assumes.  It  is  necessary, 
then,  to  stop  the  current  for  a  few  minutes,  when  the  copper  will 
at  once  go  back  into  solution,  and  to  continue  the  electrolysis  for 
a  short  time  after  adding  a  little  more  potassium  cyanide. 
*  Ber.,  34,  1607  (1901). 


RAPID  SEPARATION  OF  COPPER  FROM  SILVER        227 

When  the  deposition  of  the  silver. is  complete,  both  electrodes 
are  raised  from  the  solution,  without  breaking  the  circuit,  and 
quickly  plunged  into  a  beaker  containing  distilled  water,  after 
which  the  current  is  turned  off. 

It  is  not  advisable  to  attempt  the  electrolytic  deposition  of 
copper  from  the  solution  containing  considerable  potassium 
cyanide;  it  is  better  to  evaporate  the  solution  under  the  hood 
with  sulphuric  acid,  until  all  the  potassium  cyanide  is  decomposed, 
and  then  to  determine  the  copper  as  described  on  page  124. 

Rapid  Separation  of  Copper  from  Silver. 

In  a  potassium-cyanide  solution  of  the  two  metals,  Julia  Lang- 
ness  *  succeeded  in  effecting  a  separation  in  15  to  20  minutes. 
The  solution,  which  may  contain  about  0.12  gm.  of  silver  and  an 
equal  quantity  of  copper  in  125  cc.,  was  treated  with  2  gms.  of  po- 
tassium cyanide,  heated,  and  electrolyzed  in  a  platinum  dish  with  a 
spiral  or  sieve  anode  (Figs.  15  and  22)  making  about  600  revolu- 
tions per  minute;  the  current  was  0.4  to  0.1  ampere  at  2.5  volts. 

In  the  solution  freed  from  silver,  the  potassium  cyanide  was 
destroyed  as  described  above  and  the  copper  determined  accord- 
ing to  page  116  or  page  124. 

In  Boiling  Acetic-acid  Solution,  Sand  (p.  42)  found  it  possible 
to  deposit  silver  in  the  presence  of  copper  by  keeping  the  cathode 
potential  at  0.3  volt  (by  means  of  the  auxiliary  electrode,  p.  40), 
or  by  simply  keeping  the  potential  between  the  electrodes  below 
1.25  volts.  This  voltage  must  not  be  exceeded  even  in  the  short 
time  required  to  take  away  the  beaker  and  wash  the  deposit. 

The  solution  containing  about  0.5  gm.  of  silver  and  0.1  to  0.25  gm. 
of  copper  was  treated  with  4  or  5  cc.  of  concentrated  nitric  acid 
(or  4  cc.  of  concentrated  sulphuric  acid)  and  25  gms.  of  sodium 
acetate  and  the  boiling-hot  solution  electrolyzed  with  an  initial 
potential  difference  between  the  electrodes  of  1  volt  (correspond- 
ing to  2.8  amperes).  After  7  minutes,  when  the  potential  had 
risen  to  1.2  volts  and  the  current  strength  had  sunk  to  0.5  to  0.8 
ampere,  all  the  silver  had  been  deposited. 

In  working  with  the  auxiliary  electrode  the  solution  contained 
about  0.27  gm.  of  silver,  0.59  gm.  of  copper,  4  cc.  of  concentrated 
nitric  acid  and  25  gms.  of  sodium  acetate.  The  cathode  potential 
*  J.  Am.  Chem.  Soc.,  29,  471  (1907). 


228  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

was  kept  at  0.3  volt;    the  current  strength  was  2.7  amperes  at 
the  start  and  0.4  ampere  at  the  finish. 

The  copper  is  best  determined,  after  evaporation  with  either 
sulphuric  or  nitric  acid,  according  to  page  116  or  page  124. 

Separation  of  Copper  from  Cadmium. 

Three  methods  have  been  used  successfully  for  accomplishing 
this  separation.  1.  The  deposition  of  copper  from  a  nitric-acid 
solution.  2.  The  deposition  of  copper  from  a  sulphuric-acid 
solution.  3.  The  deposition  of  cadmium  from  a  potassium-cyanide 
solution. 

1.   Deposition  of  Copper  from  Nitric-acid  Solution. 

According  to  E.  F.  Smith  and  Wallace,*  the  solution  containing 
the  two  metals  in  100  cc.  is  acidified  with  2  cc.  of  nitric  acid 
(sp.  gr.  1.4)  heated  to  50°  and  the  copper  deposited  with  a  current 
of  ?.5  volts  and  NDioo  =  0.1  ampere.  The  electrolysis  under 
these  conditions  requires  about  3  hours. 

2.  Deposition  of  Copper  from  Sulphuric-acid  Solution. 
Heidenreich,  who  tested  the  method  proposed  by  Freudenberg, 
found  that  the  separation  succeeds  best  when  the  potential  differ- 
ence between  the  electrodes  does  not  exceed  1.85  volts.  The 
neutral  solution  of  the  two  sulphates  is  treated  with  15  cc.  of  sul- 
phuric acid  (sp.  gr.  1.09)  and  the  copper  is  deposited  with  a  current 
at  1.7  to  1.8  volts  and  NDioo  =  0.07  to  0.05  ampere,  at  the  labora- 
tory temperature.  Since  the  complete  deposition  of  the  copper 
with  such  a  weak  current  requires  a  long  time,  it  is  necessary  to 
let  the  current  run  overnight  (cf.  p.  229). 

3.   Deposition  of  Cadmium  from  Potassium-cyanide  Solution. 

The  two  methods  already  described  for  separating  copper  from 
cadmium  have  been  based  upon  the  fact  that  the  decomposition 
potential  of  copper  cations  is  less  than  that  of  cadmium  cations; 
thus  the  copper  ions  are  discharged  before  the  cadmium  ions. 

Cadmium  may  be  deposited  before  the  copper  in  a  solution  of 
the  complex  cyanides,  owing  to  the  different  degree  of  stability 
of  these  salts.  By  the  addition  of  potassium  cyanide,  both 
copper  and  cadmium  are  transformed  into  complex  anions  but 
the  Cd(CN)7  anion  is  less  stable  than  the  Cu2(CN)f"  anion. 
*  J.  Am.  Chem.  Soc.,  19,  870  (1897). 


RAPID  SEPARATION  OF  COPPER  FROM  CADMIUM      229 

The  difference  in  behavior  is  shown  by  the  fact  that  cadmium  is 
precipitated  by  hydrogen  sulphide  from  a  potassium-cyanide  solu- 
tion, whereas  under  the  same  conditions  copper  is  not,  because 
there  are  practically  no  copper  cations  in  the  solution.  The 
primary  and  secondary  dissociation  of  potassium-cadmium  cya- 
nide is  expressed  by  the  following  equilibrium  expressions,  of 
which  the  first  takes  place  almost  completely  and  the  second  to  a 
slight  extent. 

K2[Cd(CN)4]  <=±2  K+  +  [Cd(CN)  J= 
[Cd(CN)4r  ^  Cd++  +  4  (CN)-. 

The  presence  of  cadmium  cations  in  the  solution  permits  the 
electrolytic  deposition  of  the  cadmium,  inasmuch  as  the  decom- 
position potential  of  cadmium  ions  is  less  than  that  of  hydrogen 
from  an  alkaline  solution. 

If  the  solution  is  not  already  neutral,  it  is  made  so  by  the  addi- 
tion of  caustic-potash  solution,  and  potassium  cyanide  is  added  in 
sufficient  quantity  to  dissolve  the  cyanides  of  copper  and  cadmium 
which  are  first  precipitated.  After  adding  an  exces.8  of  3  or  4  gms. 
potassium  cyanide,  the  solution  is  diluted  and  electrolyzed  with 
a  current  whose  potential  is  not  allowed  to  exceed  2.6  to  2.7  volts. 

To  determine  the  copper,  the  solution  should  be  freed  from 
cyanide  and  the  electrolysis  carried  out  in  a  sulphate  or  nitrate 
solution  (cf.  p.  227). 

Rapid  Separation  of  Copper  from  Cadmium. 

According  to  D.  S.  Ashbrook,*  a  deposit  of  0.27  gm.  of  copper, 
free  from  cadmium,  can  be  obtained  in  20  minutes  if  a  solution 
of  the  two  metals,  containing  1  cc.  of  nitric  acid  (sp.  gr.  1.43),  is 
electrolyzed,  using  a  platinum  dish  as  cathode,  and  a  spiral  making 
300  to  400  revolutions  per  minute  as  anode,  with  a  current  of 
NDioo  =  3  amperes  at  4  to  5  volts. 

P.  Denso  f  proceeds  in  the  following  manner.  From  a  solu- 
tion of  the  sulphates,  containing  about  0.13  gm.  of  copper  and 
0.1  gm.  of  cadmium,  enough  sulphuric  acid  is  added  to  make  the 
acidity  correspond  to  a  double-normal  solution,  and  the  copper  is 
deposited  at  a  potential  of  not  over  2  volts.  This  maximum 
potential  is  obtained  by  connecting  the  cell  directly  to  the  poles 
of  a  single  accumulator  cell.  As  cathode,  a  gauze  electrode  serves 

*  J.  Am.  Chem.  Soc.,  26,  1285  (1904). 
t  Z.  Elektrochem.,  9,  469  (1903). 


230          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

(p.  59),  and  as  anode,  Denso  recommends  a  platinized  platinum 
wire  wound  into  a  spiral;  this  anode  is  fastened  to  the  clapper 
of  an  electric  bell  (the  bell  is  removed)  and  the  rapid  motion  of 
the  clapper  back  and  forth  serves  to  stir  the  solution.  Unques- 
tionably the  same  effect  could  be  attained  by  rotating  the  anode 
or  otherwise  stirring  the  electrolyte.  The  precipitation  of  the 
copper  requires  about  one  hour. 

In  the  solution  freed  from  copper,  which  is  concentrated  by 
evaporation  if  the  washings  have  made  it  too  dilute,  the  cadmium 
is  deposited  in  a  stationary  electrolyte  with  a  current  of  0.57  am- 
pere at  2.6  volts.  A  single  accumulator  cell  is  naturally  insuffi- 
cient in  this  case.  Each  electrolysis  requires  about  an  hour. 

Separation  of  Copper  from  Mercury. 

As  electrolyte  for  this  separation,  only  a  potassium-cyanide  solu- 
tion is  to  be  considered.  The  solution,  which  may  contain  about 
0.12  gm.  of  mercury  and  an  equal  quantity  of  copper,  is  treated 
with  2  or  3  gms.  of  pure  potassium  cyanide,  diluted  to  125  cc.  and 
electrolyzed  at  about  65°  with  a  current  of  1.5  volts  and  0.06  to 
0.08  ampere.  E.  F.  Smith  and  Spencer  found  that  the  duration 
of  the  experiment  was  so  shortened  by  heating  the  electrolyte  that 
only  2.5  to  3  hours  were  required  to  deposit  the  above  quantity 
of  mercury.  With  regard  to  heating  the  solution,  however,  the 
danger  of  losing  mercury  by  volatilization,  as  mentioned  on 
page  127,  must  be  borne  in  mind. 

Rapid  Separation  of  Copper  and  Mercury. 

H.  J.  S.  Sand,*  by  the  use  of  his  rotating  gauze  electrode  and 
the  auxiliary  electrode,  succeeded  in  depositing  mercury  in  6 
minutes  from  a  nitric-acid  solution  containing  copper.  The 
cathode  potential  was  kept  at  0.14  volt  and  the  anode  made  about 
600  revolutions  per  minute. 

Separation  of  Copper  from  Lead. 

According  to  what  was  stated  on  page  194  concerning  the  deposi- 
tion of  lead  as  peroxide  in  nitric-acid  solution,  and  on  page  124 
concerning  the  deposition  of  copper  in  a  nitric-acid  solution,  it 
is  obvious  that  it  is  possible  to  precipitate  the  two  elements 
simultaneously  by  the  electrolysis  of  a  nitric-acid  solution,  the 

*  See  p.  42. 


SEPARATION  OF  COPPER  FROM   LEAD  231 

lead  as  peroxide  upon  the  anode  and  the  copper  as  metal  upon 
the  cathode.  To  prevent  the  deposition  of  lead  upon  the  cathode 
it  is  necessary  to  provide  an  excess  of  nitric  acid  and,  unless  the 
electrolysis  is  carried  out  long  enough  to  reduce  the  excess  of 
nitric  acid,  there  is  danger  of  some  of  the  copper  not  being 
deposited.  The  author  considers  it  safer,  therefore,  to  deposit 
the  lead  peroxide  from  a  solution  so  acid  that  none  of  the  copper 
will  deposit  and  then,  after  neutralizing  the  excess  of  acid  with 
ammonia,  to  determine  the  copper  by  itself. 

The  solution  containing  20  cc.  of  nitric  acid  (sp.  gr.  1.35)  is 
diluted  to  only  75  cc.,  heated  to  60°  and  electrolyzed  with  a 
current  of  NDioo  =  1.5  to  1.7  ampere,  using  a  roughened  platinum 
dish  as  anode.  As  cathode  a  perforated,  roughened  platinum  disk, 
or  a  gauze  electrode  of  suitable  shape,  is  used  and  its  weight  is 
determined.  After  about  an  hour,  the  whole  or  greater  part  of 
the  lead  (if  0.5  gm.  was  in  the  solution)  will  be  deposited  upon 
the  dish  as  peroxide,  while  the  disk  will  show  little  or  no  copper. 
The  current  is  broken  and  the  solution  transferred  to  a  second 
weighed  platinum  dish.  The  deposited  peroxide  is  washed  with 
water  and  the  washings  added  to  the  main  solution.  The  deposit 
is  treated  as  described  on  page  194. 

To  determine  the  copper,  the  solution  is  neutralized  with  am- 
monia until  the  dark  blue  color  is  obtained  and  then  5  cc.  of  nitric 
acid  are  added.  The  platinum  dish  is  made  the  cathode  and, 
in  order  to  obtain  any  remaining  lead,  the  above-mentioned  disk 
or  gauze  electrode  is  now  used  as  anode.  It  makes  no  difference 
whether  copper  was  deposited  on  it  or  not  during  the  previous  elec- 
trolysis, for  any  copper  on  this  electrode,  which  is  now  the  anode, 
will  dissolve  and  be  deposited  upon  the  dish.  When  the  solution 
has  become  perfectly  cold,  it  is  diluted  to  about  120  or  140  cc.  and 
electrolyzed  with  a  current  of  1  to  1.2  ampere.  To  deposit  0.25 
gm.  of  copper  and  any  residual  lead,  3  or  4  hours  are  required. 

This  method  permits  a  rapid  and  accurate  quantitative  separa- 
tion of  the  two  metals  irrespective  of  the  relative  amounts  present. 

If  a  precipitate  of  lead  sulphate  is  present  in  the  solution  of 
the  two  metals  (e.g.,  on  account  of  the  oxidation  of  sulphide  ores 
with  nitric  acid)  the  analysis  often  requires  more  time,  as  the  pre- 
cipitate dissolves  in  hot  nitric  acid  more  or  less  slowly,  depending 
upon  its  physical  nature.  Formerly,  the  author  recommended 
adding  a  slight  excess  of  ammonia  and  heating  the  solution,  where- 


232          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

by  the  dense  lead  sulphate  was  transformed  into  less  dense  lead 
hydroxide.  The  ammoniacal  liquid  was  poured,  little  by  little,  into 
the  platinum  dish  containing  about  20  cc.  of  hot  nitric  acid  (sp. 
gr.  1.35)  while  stirring  constantly  with  the  electrode.  The  lead 
sulphate,  which  formed  again  upon  coming  in  contact  with  the 
acid,  either  redissolved  immediately,  or,  if  much  was  present,  it  dis- 
solved after  heating  the  acid  for  a  short  time.  The  vessel  in  which 
the  neutralization  with  ammonia  took  place  was  first  washed  with 
a  little  nitric  acid  and  then  with  pure  water,  the  washings  being 
added  to  the  main  solution,  kept  for  the  copper  determination. 

H.  J.  S.  Sand  has  found,  however,  that  it  is  unnecessary  to  bring 
the  lead  sulphate  into  solution,  and  that  the  electrolysis  may  be 
carried  out  in  the  presence  of  a  lead-sulphate  precipitate  provided 
the  electrolyte  is  stirred.  The  process  is  carried  out  in  the  follow- 
ing manner. 

Rapid  Separation  of  Copper  and  Lead. 

By  using  his  gauze  electrodes,  the  inner  of  which  acted  as  anode 
and  made  300  to  600  revolutions  per  minute,  Sand  obtained  a 
very  accurate  separation  of  0.14  gm.  of  lead  and  0.25  gm.  of  Cu  in 
a  solution  containing  some  of  the  lead  in  the  form  of  a  sulphate 
precipitate.  The  solution  was  treated  with  1  cc.  of  concentrated 
nitric  acid,  heated,  and  electrolyzed  for  5  minutes  with  a  current 
of  2  amperes.  During  this  time  the  lead  sulphate  gradually  dis- 
solved and  the  lead  peroxide  deposited  upon  the  anode.  The 
current  was  then  strengthened  to  10  amperes  and  thereby  all  the 
copper  was  deposited.  Although  the  lead  peroxide  did  not  adhere 
very  firmly,  there  was  no  loss  during  the  washing. 

It  is  noteworthy  in  such  a  case  that  hi  spite  of  the  slight  acidity 
of  the  solution  no  lead  is  deposited  upon  the  cathode  if  the  quan- 
tity of  copper  present  is  so  large  that  all  the  lead  is  deposited  as 
peroxide  upon  the  anode  before  the  precipitation  of  the  copper 
begins  (cf.  p.  197  and  Analysis  of  Commercial  Zinc,  p.  303). 

If  more  lead  than  copper  is  present  in  the  solution,  there  is  not 
enough  nitric  acid  in  Sand's  method  to  prevent  the  deposition  of 
metallic  lead  upon  the  cathode.  In  such  cases  more  nitric  acid 
must  be  added. 

A.  Fischer,  while  working  at  Aachen,  has  succeeded  in  depositing 
0.15  gm.  of  lead  and  0.27  gm.  of  copper  in  15  to  20  minutes  according 
to  the  following  experimental  conditions.  The  solution  in  the 


SEPARATION  OF  COPPER  FROM  ARSENIC  233 

platinum  dish,  which  served  as  anode,  amounted  to  120  cc.  and 
contained  20  cc.  of  nitric  acid  (sp.  gr.  1.3).  The  temperature  was 
95°,  the  current  strength  6  to  7  amperes,  the  potential  3.8  to 
3.9  volts.  The  disk  cathode  made  800  to  1000  revolutions  per 
minute. 

For  the  rapid  determination  of  the  copper,  the  greater  part  of 
the  nitric  acid  was  neutralized  with  ammonia  and  the  analysis 
carried  out  as  described  on  page  129. 

Separation  of  Copper  from  Arsenic. 

If  a  copper  solution  containing  arsenic  is  electrolyzed  by  one 
of  the  usual  methods,  toward  the  end  of  the  electrolysis  black 
specks  will  appear  upon  the  pink  copper  deposit;  if  considerable 
arsenic  is  present  the  entire  copper  deposit  becomes  covered  with 
a  black  film.  Since  nearly  all  copper  ores,  copper  alloys  and  com- 
mercial copper  contain  some  arsenic,  it  is  evident  that  the  elec- 
trolysis of  a  copper  solution  in  the  presence  of  arsenic  is  a  matter 
of  considerable  importance.  Of  the  various  methods  which  have 
been  proposed  for  keeping  the  arsenic  in  solution,  the  following 
three  methods  have  proved  to  be  the  best. 

1.  In  Sulphuric-acid  Solution.     Freudenberg  *  found  that  the 
separation  could  be  effected  in  a  solution  containing  10  to  20  cc. 
dilute  sulphuric  acid,  if  the  difference  of  potential  between  the 
electrodes  was  not  allowed  to  exceed  1 .9  volts.   In  this  way  as  much 
as  0.3  gm.  of  copper  can  be  separated  overnight  from  an  equal 
amount  of  arsenic  and  it  makes  no  difference  whether  the  latter 
element  is  present  in  the  trivalent  or  quinquevalent  condition; 
the  copper  deposit  is  free  from  arsenic. 

2.  In  Nitric-acid  Solution.     If  the  copper  is  present  in  a  nitric- 
acid  solution,  as  is  frequently  the  case  (analysis  of  alloys,  black 
copper,  etc.),  a  copper  deposit  free  from  arsenic  can  be  obtained 
in  such  a  solution  if  about  5  cc.  of  nitric  acid  are  present  in  100  cc. 
of  solution  and  the  electrolysis  is  carried  out  at  50°  to  60°  with  a 
maximum  potential  of  1.9  volts.     At  ordinary  temperatures  the 
electrolysis  requires  longer,  and  it  is  best  to  let  the  current  run 
overnight. 

As  long  as  the  arsenic  is  present  in  the  quinquevalent  condition, 
it  is  not  deposited  by  the  current  because  only  AsOf  anions  are 
present.     When  the  arsenic  acid  is  partly  reduced  to  arsenious 
*  Z.  physik.  Chem.,  12,  117  (1893). 


234  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

acid  by  the  action  of  the  current,  then  more  or  less  trivalent  As 
cations  are  present  in  the  solution  and  the  possibility  exists  for 
arsenic  to  be  deposited  at  the  cathode.  To  prevent  the  reduction, 
A.  Hollard  and  L.  Bertiaux  *  add  a  little  ferric  sulphate  to  the 
solution  (cf.  Analysis  of  Commercial  Copper). 

3.  In  Ammoniacal  Solution.  Apparently  Le  Roy  W.  McCay  f 
was  the  first  to  observe  that  it  was  possible  to  obtain  a  copper 
deposit  free  from  arsenic  by  the  electrolysis  of  an  ammoniacal 
solution.  According  to  E.  F.  Smith  the  solution  containing  about 
0.2  gm.  of  copper  is  treated  with  20  cc.  of  ammonia  (sp.  gr.  0.91)  and 
2.5  gms.  of  ammonium  nitrate.  After  diluting  to  about  125  cc.,  it 
is  electrolyzed  at  50°  to  60°  with  a  current  of  NDioo  =  0.5  ampere 
at  3.5  volts.  At  the  end  of  about  3  hours,  the  copper  is  com- 
pletely deposited  and  contains  no  arsenic. 

The  fact  that  such  a  strong  current  does  not  cause  the  deposition 
of  any  arsenic,  irrespective  of  whether  the  arsenic  is  present  as 
arsenite  or  arsenate,  is  due  to  the  fact  that  arsenic  cations  cannot 
exist  as  such  in  an  alkaline  solution;  quinquevalent  arsenic  is 
always  present  as  As04  anions,  except  perhaps  in  very  concen- 
trated hydrochloric-acid  solution,  and  trivalent  arsenic  yields  a 
small  quantity  of  the  trivalent  arsenic  cation,  As+++,  only  in  an 
acid  solution;  in  an  alkaline  solution  trivalent  arsenic  can  only 
dissociate  into  the  As03  anion. 

Freudenberg  treats  the  nitric-acid  solution  of  copper  and 
arsenic  with  ammonia  until  an  excess  of  about  30  cc.  of  10  per 
cent  ammonia  is  present  and  the  electrolysis  is  carried  out  with  a 
current  at  1.9  volts  until  the  solution  is  completely  decolorized, 
which  requires  from  6  to  8  hours. 

Rapid  Separation  of  Copper  from  Arsenic. 

D.  S.  Ashbrook,  using  Exner's  electrodes,  i.e.,  a  platinum  dish 
as  cathode  and  a  platinum  spiral  making  300  to  400  revolutions 
per  minute  as  anode,  succeeded  in  separating  0.27  gm.  of  copper 
from  an  equal  amount  of  arsenic  by  electrolyzing,  for  20  minutes, 
a  solution  to  which  1  cc.  of  concentrated  nitric  acid  (sp.  gr.  1.43) 
had  been  added.  The  volume  of  the  solution  was  about  125  cc. 
and  the  current  density  was  NDioo  =  5  amperes  at  4  to  5  volts. 

The  conditions  for  the  rapid  electrolysis  in  an  ammoniacal  solu- 

*  Bull.  soc.  chim.,  [3],  31,  900  (1904). 
t  Chem.-Ztg.,  14,  509  (1890). 


SEPARATION  OF  COPPER  FROM  BISMUTH  235 

tion  were  the  following.  The  electrolyte  contained  the  above 
quantities  of  copper  and  arsenic  in  125  cc.,  25  cc.  of  ammonia 
(sp.  gr.  0.91)  and  2.5  gms.  of  ammonium  nitrate.  The  deposi- 
tion of  the  copper  was  completed  in  15  minutes  by  using  a  current 
of  NDioo  =  5  amperes  at  7  volts. 

Separation  of  Copper  from  Aluminium,  Magnesium,  Barium, 
Strontium,  Calcium  and  the  Alkali  Metals. 

1.  In  Nitric-acid  Solution.     The  conditions  outlined  on  page  124 
serve  for  the  separation  of  copper  in  the  presence  of  salts  of  the 
above  metals. 

2.  In  Sulphuric-acid  Solution.     On  account  of  the  difficult  solu- 
bility of  the  sulphates  of  barium,  strontium  and  calcium,  the  elec- 
trolytic separation  of  copper  from  these  elements  does  not  need  to 
be  considered.     The  deposition  of  the  copper  in  the  presence  of 
aluminium,  magnesium  and  the  alkali  metals  takes  place  under 
the  conditions  described  on  page  116,  as  in  their  absence. 

Rapid    Separation   of    Copper   from   Aluminium,    Magnesium, 
Alkaline  Earths  and  Alkali  Metals. 

1.  In  Nitric-acid  Solution.     According  to  Ashbrook,*  who  only 
attempted  the  separation  of  copper  from  aluminium  and  from 
magnesium,  it  is  possible  to  deposit  0.27  gm.  of  pure  copper  in  the 
presence  of  about  the  same  quantity  of  aluminium  or  magnesium, 
if  the  solution  is  treated  with  1  cc.  of  concentrated  nitric  acid, 
diluted  to  125  cc.  and  electrolyzed  for  20  minutes  with  a  current 
of   NDioo  =  3  amperes  at  4  to  5  volts.      The  spiral  anode  (cf. 
p.  54)  is  given  a  velocity  of  300  to  400  revolutions  per  minute. 

2.  In  Sulphuric-acid  Solution.     If,  instead  of  the  nitric  acid, 
0.1  cc.  of  concentrated  sulphuric  acid  is  added,  the  electrolysis  of 
the  above  quantity  of  copper  in  the  presence  of  magnesium  and 
aluminium  requires  10  minutes  with  a  current  of  NDioo  =  4  or 
5  amperes.f 

Separation  of  Copper  from  Bismuth. 

These  two  metals  stand  close  to  one  another  in  the  potential 
series  and  thus  it  is  obviously  impossible  to  effect  a  separation  in 

*  J.  Am.  Chem.  Soc.,  26,  1285  (1904). 

t  The  voltage  was  given  as  from  1  to  4.8  volts  in  the  original  article,  and  in 
E.  F.  Smith's  book  it  is  given  as  14  to  8  volts.  There  is  evidently  some  mis- 
take in  each  case. 


236          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

an  acid  solution.  Even  in  solutions  of  the  complex  salts,  the  sep- 
aration is  associated  with  difficulties.  In  electro-analysis  the 
only  question  that  has  received  attention  is  the  prevention  of  the 
deposition  of  bismuth  when  present  in  small  quantities  in  copper 
solutions  (from  ores  or  crude  copper).  For  this  purpose  A.  Hoi- 
lard  and  L.  Bertiaux  have  devised  a  simple  method  which  consists 
in  adding  a  little  finely  powdered  lead  sulphate  to  the  substance 
(copper,  alloy,  or  ore)  while  dissolving  it  in  nitric  acid.  The 
adherent  deposit  of  lead  peroxide  on  the  anode  causes  bismuth 
peroxide  to  deposit  and  adhere  there,  so  that  no  metallic  bismuth 
reaches  the  cathode. 

It  is  not  advisable  to  let  the  heavy  lead  sulphate  remain  in  con- 
tact with  the  copper  deposit  while  the  latter  is  being  formed  and 
for  this  reason  it  is  better  to  carry  out  the  electrolysis  with  a  stirred 
electrolyte.  (For  more  specific  details,  see  Analysis  of  Commercial 
Copper.) 

If  small  quantities  of  antimony  are  present  in  the  solution,  they 
will  also  adhere  as  oxide  to  the  lead-peroxide  precipitate. 

Separation  of  Copper  from  Chromium. 

The  conditions  described  for  the  separation  of  copper  from 
aluminium  hold  here  both  for  stationary  and  for  moving  electro- 
lytes (cf.  p.  235). 

For  the  rapid  separation  in  sulphuric-acid  solution,  Ashbrook 
recommends  starting  the  analysis  with  3  amperes  and  gradually 
increasing  the  current  to  5  amperes.  In  a  rapid  separation  from 
nitric-acid  solution,  the  results  are  a  little  too  high  if  the  current  is 
more  than  3  amperes. 

Separation  of  Copper  from  Antimony. 

Although  small  quantities  of  antimony  remain  in  solution  during 
the  deposition  of  copper  from  ammoniacal  solution  (p.  129), 
for  the  separation  of  larger  quantities  of  antimony  from  copper  a 
different  method  must  be  chosen.  E.  F.  Smith  and  D.  L.  Wallace  * 
add  to  the  solution  containing  0.1  gm.  of  each  metal,  or  even  twice 
as  much  antimony  in  the  quinquevalent  condition,  8  gms.  of  tar- 
taric  acid  and  30  cc.  of  ammonia  (sp.  gr.  0.91).  The  resulting 

*  Z.  anorg.  Chem.,  4,  273  (1893);  see  also  S.  C.  Schmucker,  Z.  anorg.  Chem., 
6,  199  (1894). 


SEPARATION  OF  COPPER  FROM  IRON  237 

solution  is  heated  to  50°  and  electrolyzed  at  a  volume  of  150  cc. 
with  a  current  of  NDioo  =  0.08  to  0.1  ampere  at  1.8  to  2  volts. 

The  solution,  after  being  freed  from  copper,  is  converted  into 
sulpho  salt  and  the  antimony  determined  according  to  page  158.* 

Concerning  the  deposition  of  pure  copper  in  the  presence  of 
antimony,  consult  the  article  on  Commercial  Copper,  in  Part  IV. 

Separation  of  Copper  from  Iron. 

In  a  nitric-acid  solution  the  deposition  of  copper,  free  from  iron, 
is  effected  under  the  conditions  described  on  page  124.  If  larger 
quantities  of  iron  are  in  solution,  the  ferric  nitrate  exerts  a  solvent 
effect  upon  the  deposited  copper;  at  all  events  the  time  required 
is  longer.  Inasmuch  as  large  quantities  of  nitric  acid  hinder  the 
deposition  of  copper,  Hollard  and  Bertiaux  recommend  the  reduc- 
tion of  the  excess  nitric  acid  by  the  addition  of  a  saturated  solution 
of  sulphurous  acid;  an  excess  of  this  reagent  must  be  avoided  as 
otherwise  copper  sulphide  may  be  precipitated. f 

For  the  determination  of  the  iron,  the  solution  after  the  elec- 
trolysis is  evaporated  with  concentrated  sulphuric  acid  until  the 
nitric  acid  is  all  expelled,  the  free  sulphuric  acid  is  neutralized 
with  ammonia,  8  gms.  of  ammonium  oxalate  are  added,  and  the 
iron  is  determined  electrolytically  as  described  on  page  183. 

The  separation  of  copper  from  iron  takes  place  more  satisfac- 
torily in  a  sulphuric-acid  solution,  because  the  ferric  salt  is  reduced 
to  ferrous  salt  during  the  electrolysis  and  the  above-mentioned 
solvent  effect  is  lost.t 

The  iron  determination  is  carried  out  as  described  above,  after 
concentrating  the  solution  by  evaporation. 

*  Another  method  for  separating  copper  and  antimony  is  described  by 
Puschin  and  Trechzinsky,  Z.  Elektrochem.,  14,  47  (1907). 

t  For  the  determination  of  copper  in  materials  rich  in  iron,  see  page  293. 

t  This  is  true  only  when  the  work  is  carried  out  at  ordinary  temperatures. 
If  the  deposition  of  the  copper  takes  place  with  a  potential  of  2  volts  (p.  119) 
and  at  a  temperature  of  75°,  then  0.15  gm.  of  iron  in  100  cc.  of  solution  can 
prevent  the  quantitative  deposition  of  the  copper,  because  at  this  temper- 
ature the  ferric  salt  formed  at  the  anode  diffuses  quickly  to  the  cathode  and 
is  reduced  to  ferrous  salt  by  the  current  more  readily  than  cupric  ions  are 
discharged.  At  ordinary  temperatures,  however,  the  presence  of  even  0.6  gm. 
iron  in  100  cc.  does  not  hinder  the  deposition  of  0.15  gm.  copper,  because  in 
this  case  the  diffusion  takes  place  more  slowly.  (F.  Foerster,  Z.  angew. 
Chem.,  19,  1895  (1906)). 


238          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

In  ammoniacal  solution  copper  can  be  separated  from  large 
quantities  of  iron  by  the  method  of  G.  Vortmann.*  The  iron  is 
oxidized  to  the  ferric  condition  by  nitric  acid,  ammonium  sulphate 
is  added  and  the  iron  precipitated  by  an  excess  of  ammonia. 
Without  filtering  off  the  precipitated  ferric  hydroxide,  the  copper 
is  determined  with  a  current  of  NDioo  =  0.1  to  0.06  ampere.  It 
is  advisable  in  this  case,  as  in  all  analyses  carried  out  in  the  presence 
of  a  substance  in  suspension,  to  use  a  cylindrical  or  conical  cathode 
rather  than  a  platinum  dish;  because  the  long  contact  of  the  pre- 
cipitate with  the  deposit  may  give  rise  to  inaccuracies. f  It  is 
well  to  carry  out  the  work  with  a  stirred  electrolyte. 

The  method  given  in  previous  editions  of  this  book,  which  con- 
sisted in  using  an  ammonium-oxalate  solution  with  oxalic,  tartaric 
or  acetic  acid,  has  no  advantages  over  the  methods  already  de- 
scribed. 

Rapid  Separation  of  Copper  from  Iron.     « 

According  to  D.  S.  Ashbrook,  the  same  conditions  are  necessary 
as  in  the  separation  of  copper  from  aluminium  in  nitric-  or  sul- 
phuric-acid solution  (cf.  p.  235). 

To  prevent  the  impeding  action  of  nitric  acid,  A.  Fischer  recom- 
mends the  addition  of  0.5  to  1  gm.  of  hydrazine  sulphate  toward 
the  end  of  the  electrolysis.  Fischer  used  a  platinum  dish  as 
cathode  and  a  disk  making  1000  to  1200  revolutions  per  minute 
as  anode.  The  solution  contained  1  cc.  of  concentrated  nitric  acid 
and  was  electrolyzed  at  95°  at  a  volume  of  125  cc.  with  a  current 
of  3.5  to  4  amperes  at  6.3  to  8.5  volts.  Under  these  conditions, 
about  0.27  gm.  of  copper  can  be  separated  from  0.2  gm.  of  iron  in 
20  to  25  minutes. 

In  Potassium-cyanide  Solution.  The  separation  of  copper  from 
iron  in  ammoniacal  solution  depends  upon  the  removal  of  the 
ferric  ions  by  precipitation,  but  the  separation  in  a  potassium- 
cyanide  solution  depends  upon  the  removal  of  ferrous  and  ferric 
ions  by  converting  them  into  the  extremely  stable  ferrocyanide 
and  ferricyanide  ions.  In  either  case  no  iron  cations  are  present 
in  the  solution.  The  cuprocyanide  anion  is  less  stable  than  the 
complex  iron  anions,  and  undergoes  a  secondary  dissociation  to 

*  Monatsh.  Chem.,  14,  552  (1893). 

t  B.  Neumann  first  called  attention  to  this  source  of  error  and  A.  Thiel 
has  confirmed  it,  Z.  Elektrochem.,  14,  205  (1908);  cf.  p.  186. 


SEPARATION  OF  COPPER  FROM   MAGNESIUM  2S9 

some  extent,  forming  a  few  copper  cations,  and  the  extent  to  which 
this  secondary  dissociation  takes  place  is  greater  in  proportion 
as  less  potassium  cyanide  is  added  (cf.  p.  228).  Moreover,  the 
potassium  cyanide  is  decomposed  by  the  current  and  thus  the 
tendency  for  the  complex  cuprocyanide  to  dissociate  becomes  in- 
creased while,  at  the  same  time,  the  more  stable  ferrocyanide  or 
ferricyanide  anions  are  not  decomposed  by  the  current  used.  If, 
furthermore,  only  a  little  potassium  cyanide  is  used  the  salt  does 
not  attack  the  anode  *  causing  deposition  of  platinum,  together 
with  the  copper,  upon  the  cathode.  Such  an  attack  by  the 
potassium  cyanide  is  also  prevented  by  the  addition  of  ammonia. 

On  the  basis  of  these  facts,  A.  L.  Flanigen  f  successfully  accom- 
plished the  separation  of  copper  from  iron  under  the  following 
conditions.  To  the  solution  containing  about  0.2  gm.  copper, 
1.5  gms.  of  pure  potassium  cyanide  and  10  cc.  of  ammonia  (sp. 
gr.  0.93)  were  added,  and  after  heating  to  65°  the  copper  was 
deposited  with  a  current  of  NDioo  =  8  to  10  amperes  at  10  volts. 
The  anode  made  about  400  revolutions  per  minute  and  the  analy- 
sis required  10  minutes.  It  makes  no  difference  whether  the  iron 
content  is  greater  or  less  than  the  copper  content. 

If  it  is  desired  to  determine  the  iron  electrolytically,  this  method 
is  a  tedious  one  because  it  is  necessary  to  destroy  the  complex 
anions,  by  evaporating  with  sulphuric  acid,  before  going  on  with 
the  analysis. 

Separation  of  Copper  from  Manganese. 
The  simultaneous  deposition  of  copper  upon  the  cathode  and 
manganese  dioxide  upon  the  anode  gives  uncertain  results;  for 
one  reason  because  the  conditions  necessary  for  the  deposition  of 
the  manganese  (cf.  p.  197)  give  rise  to  poor  deposits  of  copper,  and 
for  another  reason  because  the  presence  of  mineral  acids,  which 
favor  the  formation  of  good  copper  deposits,  tend  to  prevent  the 
complete  deposition  of  manganese  dioxide.  It  is  necessary,  there- 
fore, to  deposit  the  copper  as  described  on  page  116,  and  then,  in 
case  it  is  desired  to  determine  manganese  electrolytically,  trans- 
form the  solution  into  one  suitable  for  such  a  determination. 

Separation  of  Copper  from  Magnesium. 
See  Separation  of  Copper  from  Aluminium,  etc.,  on  page  235. 

*  F.  Spitzer,  Z.  Elektrochem.,  11,  407  (1905). 
t  J-  Am.  Chem.  Soc.,  29,  455  (1907). 


240          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Separation  of  Copper  from  Cobalt  and  Nickel. 

This  separation,  which  is  of  importance  in  the  analysis  of  such 
alloys  as  German  silver,  (Cu,  Ni,  Zn),  can  take  place  with  station- 
ary electrolytes  either  in  sulphuric-  or  nitric-acid  solutions;  by 
the  rapid  method  good  results  are  obtained  only  in  nitric-acid 
solution. 

Deposition  of  the  Copper  from  Sulphuric-acid  or  from  Nitric-acid 
Solution.  The  solution,  containing  about  0.25  gm.  of  copper  and 
0.2  gm.  of  nickel  or  cobalt,  is  treated  with  3  cc.  of  concentrated- 
sulphuric  acid,  or  with  5  cc.  of  concentrated  nitric  acid,  diluted  to 
150  cc.  and  the  copper  deposited,  without  heating  the  solution, 
with  a  current  of  1  ampere.  The  analysis  requires  about  3  hours 
(cf.  p.  116  et  seq.). 

In  the  solution  freed  from  copper,  the  nickel  or  cobalt  can  be 
deposited  by  the  method  described  on  page  185. 

According  to  P.  Denso,*  copper  can  be  separated  from  cobalt 
and  nickel  by  keeping  the  voltage  within  certain  limits.  The 
solution,  containing  0.13  gm.  copper  and  0.1  gm.  nickel  in  the 
form  of  sulphates,  is  made  0.2  normal  with  acid  and  a  current  is 
used  of  which  the  potential  cannot  rise  above  2  volts,  e.g.,  the  cur- 
rent from  a  single  accumulator  cell.  Denso  recommends  the  use 
of  a  platinized  rotating  anode.  The  deposition  of  the  copper  is 
complete  at  the  end  of  2  hours  and  45  minutes. 

The  nickel  or  cobalt  can  be  determined  in  the  solution,  freed 
from  copper,  after  adding  an  excess  of  ammonia;  or  the  solution 
is  nearly  neutralized  with  sodium  carbonate,  and  the  barely  acid 
solution  electrolyzed  with  a  current  of  4  volts  (two  storage  cells 
in  series).  Platinizing  and  rotating  the  anode  are  desirable. 

Rapid  Separation  of  Copper  from  Nickel. 

This  method  gives  good  results  only  in  the  presence  of  nitric 
acid.  F.  F.  Exner  f  carries  out  the  separation  in  the  following 
manner.  The  solution,  containing  about  0.25  gm.  of  each  metal 
in  125  cc.,  is  treated  with  0.24  cc.  of  concentrated  nitric  acid  and 
3  gms.  of  ammonium  nitrate.  The  electrolysis  is  carried  out  with 
a  platinum  dish  and  rotating  spiral  anode  (about  600  revolutions 
per  minute)  with  a  current  of  NDi00  =  4  amperes  at  5  volts.  The 
deposition  of  the  copper  requires  about  15  minutes.  The  stated 

*  Z.  Elektrochem.,  9,  469  (1903).      t  J-  Am.  Chem.  Soc.,  26,  905  (1903). 


SEPARATION  OF  COPPER  FROM   MOLYBDENUM         241 

quantity  of  nitric  acid  has  been  found  most  favorable.  The  solu- 
tion is  heated  nearly  to  boiling  before  beginning  the  electrolysis 
and  it  is  kept  hot  by  the  heating  effect  of  the  current. 

A.  Fischer,  in  the  author's  laboratory,,  confirmed  the  data  of 
Exner  but  found  it  better  to  give  the  anode  a  speed  of  1000 
revolutions  per  minute. 

» 
Analysis  of  a  Nickel  Coin. 

Exner  carried  out  a  complete  analysis  of  a  coin  containing 
copper,  nickel  and  a  little  iron  in  2  hours  and  30  minutes  by  the 
following  method. 

The  coin,  weighing  4.925  gms.,  was  dissolved  in  20  cc.  of  con- 
centrated nitric  acid  diluted  with  an  equal  volume  of  water,  the 
solution  exactly  neutralized  with  ammonia  and  diluted  up  to  the 
mark  in  a  250-cc.  calibrated  flask.  One-tenth  of  the  solution  was 
treated  with  3  gms.  of  ammonium  sulphate,  diluted  to  125  cc.,  and 
electrolyzed  hot  (see  above)  with  a  current  of  NDioo  =  5  amperes 
at  5.5  volts.  The  copper  was  deposited  in  20  minutes. 

The  nickel  was  next  precipitated  by  caustic  soda  and  bromine 
water,  the  precipitated  nickelic  hydroxide  (and  ferric  hydroxide) 
filtered  off  and  dissolved  in  2  cc.  of  concentrated  sulphuric  acid 
and  water.  The  resulting  solution  was  diluted  to  125  cc.,  after  the 
addition  of  30  cc.  of  strong  ammonia,  and  electrolyzed  hot  with 
a  current  of  NDioo  =  6  amperes  at  5  volts.  The  nickel  was 
deposited  in  20  minutes. 

The  solution  still  contained  ferric  hydroxide  in  suspension.  It 
was  filtered  off,  dried,  ignited  and  weighed. 

In  the  above  case  the  ammonium  nitrate,  formed  by  the  neutral- 
ization of  the  nitric  acid,  serves  to  make  the  nearly  neutral  solu- 
tion a  better  conductor. 

Separation  of  Copper  from  Molybdenum  and  from 
Tungsten. 

The  deposition  of  copper  in  the  presence  of  one  of  the  above 
metals  can  be  effected  in  a  potassium-cyanide  solution.  About 
1.5  gms.  of  potassium  cyanide  are  dissolved  in  150  cc.  of  the 
solution  and  the  electrolysis  is  carried  out  at  60°  with  a  current 
of  NDioo  =  0.28  ampere  at  4  volts.  After  5  or  6  hours,  all  the 
copper  is  deposited. 


242  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Separation  of  Copper  from  Palladium  and  from  Platinum. 

If  1.5  gms.  of  potassium  cyanide  and  5  gms.  of  ammonium  car- 
bonate are  added  to  125  cc.  of  solution,  and  the  electrolysis  is 
carried  out  at  70°  with  a  current  of  ND10o  =  0.2  ampere  at  2  to 
2.5  volts,  the  copper  will  be  deposited  in  5  or  6  hours. 

Rapid  Separation  of  Copper  from  Platinum. 

J.  Langness  *  succeeded  in  depositing  0.13  gm.  of  copper  free 
from  platinum  in  35  minutes  under  the  following  conditions.  The 
solution  contained  3  gms.  of  potassium  cyanide  and  10  cc.  of  am- 
monia (cf.  p.  239)  and  was  electrolyzed  hot  with  a  current  of  3 
to  3.5  amperes  at  5  volts  potential. 

Separation  of  Copper  from  Selenium  (E.  F.  Smith)  f. 

In  Sulphuric-  or  Nitric-acid  Solution.  To  the  solution  containing 
about  0.08  gm.  copper  and  0.25  gm.  sodium  selenate  in  150  cc., 
1  cc.  of  concentrated  sulphuric,  or  nitric  acid  is  added  and  after 
heating  to  65°  the  copper  is  deposited  with  a  current  of  NDioo  = 
0.05  to  1  ampere  at  2.25  volts. 

In  Potassium-cyanide  Solution.  The  solution,  containing  1  gm. 
of  potassium  cyanide  in  150  cc.,  is  electrolyzed  with  a  current  of 
ND10o  =  0.2  ampere  at  4  volts.  In  both  cases  the  electrolysis 
requires  about  5  hours. 

Separation  of  Copper  from  Tellurium. 

In  Nitric-acid  Solution.  (D.  L.  Wallace.)  To  100  cc.  of  solu- 
tion containing  about  0.15  gm.  copper  and  0.11  gm.  tellurium, 
0.5  cc.  of  concentrated  nitric  acid  is  added  and  the  solution  elec- 
trolyzed with  NDioo  =  0.1  ampere  at  2.06  volts.  The  deposition 
of  the  copper  requires  5  hours. 

In  Sulphuric-acid  Solution.  E.  F.  Smith  J  deposited  the  copper 
in  6  hours  under  the  following  conditions.  Used  0.074  gm.  copper, 
0.2  gm.  sodium  tellurate,  1  cc.  concentrated  sulphuric  acid;  volume 
150  cc.:  temperature  65°;  NDioo  =  0.05  to  0.1  ampere;  2  to  2.25 
volts. 

Separation  of  Copper  from  Tungsten. 

See  Separation  from  Molybdenum  on  page  241. 

*  J.  Am.  Chem.  Soc.,  29,  471  (1907).  t  Ibid.,  26,  895  (1903). 

J  Ibid.,  26,  895  (1903). 


SEPARATION  OF  COPPER  FROM  TIN  243 

Separation  of  Copper  from  Uranium. 

In  Nitric-acid  Solution.  Copper  is  deposited  in  3  hours  under 
the  following  conditions.  Volume  150  cc.  with  0.5  cc.  concen- 
trated sulphuric  acid;  temperature  60°;  NDioo  =  0.14  to  0.27 
ampere;  2  to  2.4  volts. 

In  Sulphuric-acid  Solution.  Volume  150  cc. ;  2  cc.  concentrated 
sulphuric  acid;  temperature  55°;  NDioo  =  0.16  ampere;  2  volts; 
time,  4  hours. 

Rapid  Separation  of  Copper  from  Uranium. 

In  nitric-  or  sulphuric-acid  solution,  the  separation  is  effected 
in  the  same  way  as  in  the  separation  of  copper  from  aluminium 
(Ashbrook,  see  p.  235). 

Separation  of  Copper  from  Zinc. 

In  Nitric-acid  Solution.  M.  Heidenreich  *  has  tested  the  con- 
ditions proposed  by  E.  F.  Smith  and  Wallace  and  found  good 
results  as  follows.  Volume  120  cc.;  4  cc.  nitric  acid  (sp.  gr.  1.3); 
potential  of  the  bath  not  more  than  1.4  volts;  time,  18  to  20  hours. 

In  Sulphuric-acid  Solution,  the  separation  can  be  carried  out 
under  the  conditions  given  for  the  separation  of  copper  from 
aluminium,  or  from  nickel  (pp.  235,  240).  , 

The  separation  in  an  oxalic-acid  solution  has  no  advantages 
over  these  methods. 

Rapid  Separation  of  Copper  from  Zinc. 

In  Nitric-acid  Solution.  Exner  states  that  the  conditions  may 
be  made  the  same  as  in  the  separation  of  copper  from  nickel 
(p.  206),  with  the  difference  that  the  potential  is  9  volts. 

In  Sulphuric-acid  Solution.  D.  S.  Ashbrook  f  ootained  a  satis- 
factory separation  under  the  following  conditions.  Used  0.29  gm. 
Cu,  0.25  gm.  Zn;  volume  125  cc.;  1  cc.  concentrated  H2S04; 
NDioo  =  3  amperes,  gradually  raised  to  5  amperes;  5  volts;  time, 
10  minutes.  A  platinum  dish  was  used  as  cathode  and  the  spiral 
anode  made  600  revolutions  per  minute. 

Separation  of  Copper  from  Tin. 

A  solution  containing  these  two  metals  is  seldom  obtained  in 
the  course  of  an  ordinary  analysis;  as  a  rule  the  tin  is  converted 
*  Ber.,  28,  1585  (1895).  f  J-  Am.  Chem.  Soc.,  26,  1287  (1904). 


244  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

into  insoluble  metastannic  acid  by  the  action  of  nitric  acid  upon 
the  alloy  or  ore,  while  the  copper  dissolves  as  nitrate.  In  this 
case  it  is  unnecessary  to  filter  the  solution.  If  the  electrolysis  is 
to  be  carried  out  with  a  stationary  electrolyte,  a  platinum  cone  or 
gauze  cathode  is  used,  as  mentioned  on  page  123. 

It  is  a  well-known  fact  that  the  insoluble  metastannic  acid  in- 
variably contains  traces  of  copper.  To  free  the  precipitate  from 
this  copper,  the  cathode  is  removed  from  the  electrolyte  (it  may 
be  laid  upon  a  watch  glass  without  washing),  the  metastannic  acid 
is  stirred  up  into  the  liquid  which  is  heated  and  allowed  to  settle; 
the  cathode  is  then  replaced  and  the  rest  of  the  copper  deposited. 

In  the  technical  analysis  of  bronze,  a  sufficiently  pure  metastan- 
nic acid  is  obtained  by  treating  the  alloy  with  50  cc.  of  nitric  acid 
(sp.  gr.  1.2),  evaporating  just  to  dryness  (without  baking  the 
residue)  and  treating  with  successive  portions  of  10  cc.  concen- 
trated nitric  acid  in  50  cc.  of  water.  Finally  the  solution  is  heated 
to  boiling  and  the  precipitate  allowed  to  settle  (cf.  Arfalysis  of 
Bronze). 

For  the  deposition  of  copper  in  the  presence  of  metastannic  acid, 
the  use  of  a  gauze  cathode  and  rotating  anode  is  advisable  and  in 
this  way  there  is  little  danger  of  the  deposited  copper  being  con- 
taminated with  inclusions  of  metastannic  acid. 


SILVER. 

Separation  of  Silver  from  Aluminium. 

If  the  silver  is  deposited  from  a  nitric-acid  solution  as  described 
on  page  131,  the  aluminium  remains  in  solution. 

Rapid  Separation  of  Silver  from  Aluminium. 

Ashbrook,  using  a  rotating  spiral  anode  (cf.  p.  229),  was  able  to 
obtain  a  quantitativ3  deposition  of  the  silver,  but  the  metal  ad- 
hered badly  to  the  cathode  and  was  hard  to  wash  without  loss. 
The  conditions  were:  volume  =  125  cc.;  1  cc.  HNOs  (sp.  gr.  1.43); 
NDioo  =  3  amperes,  at  3.5  volts;  time  =  15  minutes. 

Under  the  same  conditions,  silver  may  be  separated  from  lead, 
chromium,  iron,  cadmium,  cobalt,  magnesium,  manganese,  nickel 
and  zinc. 


SEPARATION  OF  SILVER  FROM  ANTIMONY  245 

Separation  of  Silver  from  Antimony. 

The  methods  based  on  the  use  of  graded  potentials  as  devised 
by  H.  Freudenberg  *  have  been  tested  and  modified  by  A. 
Fischer.f 

Deposition  of  Silver  in  Nitric-Tartaric-acid  Solution. 

The  presence  of  the  tartaric  acid,  which  is  necessary  to  keep  the 
antimony  in  solution,  has  a  favorable  effect;  it  lessens  the  resis- 
tance of  the  bath  and  the  discharge  potential  of  the  silver  from 
such  a  solution  lies  about  0.3  volt  higher  than  from  a  solution  con- 
taining only  nitric  acid.  Thus,  for  depositing  the  last  traces  of 
the  silver,  it  is  perfectly  safe  to  increase  the  potential  up  to  1.45 
volts.  The  solution  containing  from  0.24  to  0.29  gm.  of  silver  and 
0.18  to  0.34  gm.  of  quinquevalent  antimony  is  treated  with  5  gms. 
of  tartaric  acid  and  2  cc.  of  nitric  acid  (sp.  gr.  1.4),  diluted  to  160  cc. 
and  heated  to  50°  or  60°.  The  electrolysis  is  first  carried  out*  for 
3  hours  at  a  potential  of  1.35  volts  corresponding  to  0.12  ampere; 
then,  when  most  of  the  silver  has  been  deposited,  the  potential  is 
increased  to  1.4  to  1.45  volts.  At  the  end  of  8  or  9  hours  the 
deposition  of  the  silver  is  complete  and  no  antimony  will  be 
found  with  the  silver  because  under  the  above  conditions  anti- 
mony is  not  reduced  to  the  trivalent  condition  until  the  potential 
of  the  current  reaches  between  1'5  and  1.6  volts.  The  current 
strength  then  falls  to  0.02  ampere.  The  tartaric  acid,  owing  to 
its  reducing  action,  prevents  the  deposition  of  silver  peroxide  upon 
the  anode  and  thus  the  addition  of  alcohol  is  unnecessary  for 
this  purpose.  If  the  electrolysis  is  conducted  at  the  laboratory 
temperature,  the  analysis  requires  nearly  18  hours.  The  deposit 
must  be  washed  while  the  current  is  passing. 

To  prepare  the  electrolyte  for  the  antimony  determination,  it 
is  merely  necessary  to  concentrate  by  evaporating  and,  after  neu- 
tralizing with  sodium  hydroxide,  to  treat  with  80  cc.  of  a  saturated 
solution  of  sodium  sulphide.  A  little  potassium  cyanide  is  added 
(cf.  p.  159)  and  the  electrolysis  carried  out  at  60°  to  70°  with  1  to 
1.5  amperes  and  1.3  to  1.6  volts. 

Deposition  of  the  Silver  from  Potassium-Cyanide  Solution. 
The  antimony  must  be  present  in  the  quinquevalent  condition 
and  the  reason  for  this  will  be  made  clear.     In  a  solution  of  potas- 

*  Z.  phys.  Chem.,  12,  109  (1893).  t  Ber.,  36,  3345  (1903). 


246          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

slum-silver  cyanide  containing  about  0.3  per  cent  silver  and  some 
tartaric  acid,  the  deposition  of  silver  begins  with  an  electromotive 
force  lying  between  1.9  and  2  volts.  This  must  be  raised  to  2.6 
volts  at  the  last  in  order  to  precipitate  the  last  traces  of  silver, 
within  a  reasonable  length  of  time.  In  such  a  solution  the  deposi- 
tion of  antimony  in  the  quinquevalent  condition  does  not  begin 
until  the  potential  of  the  current  reaches  2.6  volts,  but  it  takes 
place  between  2  and  2.1  volts  if  the  antimony  is  present  in  the 
trivalent  condition.  It  is  thus  impossible  to  carry  out  the  sepa- 
ration if  trivalent  antimony  is  present. 

The  solution  must  contain  0.5  to  1  gm.  of  tartaric  acid  and  3  to 
5  gms.  of  potassium  cyanide  in  150  to  180  cc.  It  is  heated  to 
40°  or  50°  and  electrolyzed  with  a  current  whose  potential  reaches 
2.5  volts  but  must  not  rise  above  2.6  volts.  The  current  strength 
is  0.18  ampere  at  the  start  and  falls  toward  the  end  of  the 
reaction  to  about  0.04  ampere.  The  analysis  requires  about  8 
hours  (19  to  20  hours  at  the  ordinary  temperature).  Trie  wash- 
ing of  the  deposit  can  be  accomplished  after  the  circuit  is  broken 
if  it  is  done  quickly. 

In  the  solution  concentrated  by  evaporation,  the  antimony  can 
be  determined  as  described  above  after  the  addition  of  a  little 
more  cyanide. 

As  regards  the  choice  between  the  above  two  methods,  the  latter, 
on  account  of  the  limited  solubility  of  the  antimonate,  is  to  be 
recommended  when  less  antimony  than  silver  is  present. 

The  potassium  cyanide  must  be  pure  and  free  from  cyanate, 
being  dissolved  freshly  before  each  analysis.  An  impure  cyanide 
will  cause  the  formation  of  an  ill-looking,  yellowish-green  silver 
deposit  and  its  weight  will  be  too  high. 

Separation  of  Silver  from  Arsenic. 

The  separation  of  silver  in  potassium-cyanide  solution  succeeds 
under  the  conditions  described  for  the  separation  of  silver  from 
antimony  and  the  arsenic  must  be  present  in  the  quinquevalent 
condition. 

Separation  of  Silver  from  Lead. 

It  has  been  shown  (p.  131)  that  slight  acidity  and  a  low-potential 
current  are  necessary  for  the  formation  of  a  good  silver  deposit 


SEPARATION  OF  SILVER  FROM  LEAD  247 

and  that  the  best  deposits  of  lead  peroxide  are  obtained  in  a 
strongly  acid  solution  with  a  high  voltage  (p.  194).  Arth  and 
Nicolas  *  take  advantage  of  this  contrasted  behavior  for  the  deter- 
mination of  small  quantities  of  silver  in  the  presence  of  much  lead. 
Since  the  volume  of  the  solution  is  relatively  large,  on  account  of 
taking  a  large  sample  for  the  analysis,  the  electrolysis  is  conducted 
in  a  beaker  with  gauze  electrodes  (p.  59).  According  to  the 
silver  content,  from  2.5  to  100  gms.  of  the  lead  alloy  are  dissolved 
in  nitric  acid  and  the  excess  of  acid  is  removed  by  evaporating  to 
dryness,  because  in  carrying  out  the  electrolysis  it  is  necessary  to 
regulate  closely  the  quantity  of  acid  present.  The  dry  residue 
is  dissolved  in  water  and  the  volume  of  the  solution  is  adjusted 
about  as  follows:  130  cc.  for  2.5  gms.  of  alloy,  300  cc.  for  5  to 
20  gms.,  500  cc.  for  40  to  100  gms.  One  per  cent  by  volume  of 
concentrated  sulphuric  acid  is  added  and  6  cc.  of  95  per  cent 
alcohol.  If  less  acid  were  added,  some  lead  is  likely  to  precipi- 
tate with  the  silver  upon  the  cathode.  The  solution  is  heated  to 
55°  or  60°  and  electrolyzed  with  a  maximum  potential  of  1.1  volts. 
A  current  of  higher  voltage  than  this  may  give  rise  to  spongy 
deposits.  At  the  laboratory  temperature  the  electrolysis  would 
require  a  long  time  but  at  60°,  7  hours  is  usually  enough.  If  the 
volume  of  the  solution  is  large,  the  current  must  be  allowed  to 
flow  a  little  longer  and  the  solution  stirred  often  to  hasten  the 
migration  of  the  silver  to  the  cathode. 

The  authors  have  published  numerous  results  to  show  the  value 
of  the  method.  The  quantity  of  silver  present  in  the  alloys 
tested  varied  from  0.01 1  gm.  Ag  and  2.5  gms.  Pb  to  0.001  gm.  Ag 
and  100  gms.  Pb.  For  the  determination  of  still  smaller  quan- 
tities of  silver,  an  even  larger  weight  of  alloy  may  be  taken  and  a 
larger  volume  of  solution  used  but  the  percentage  of  acid  present 
should  be  kept  the  same.  In  the  more  concentrated  solutions, 
a  deposit  of  lead  peroxide  is  often  noticed  on  the  anode  but  this 
has  no  effect  upon  the  silver  determination.  If  the  gain  in  weight 
at  the  cathode  is  too  small  to  determine  with  certainty,  the  same 
cathode  may  be  used  in  the  analysis  of  another  portion  of  the 
alloy. 

Small  quantities  of  copper  or  bismuth,  often  present  in  commer- 
cial lead,  do  not  cause  any  difficulty  as  they  are  not  deposited  at 
the  low  voltage  used. 

*  Bull.  soc.  chim.,  [3],  29,  633  (1903). 


248          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Rapid  Separation  of  Silver  from  Lead. 

The  difficulty  of  determining  large  quantities  of  silver  in  the 
presence  of  lead  lies  in  the  danger  of  some  silver  peroxide  being 
deposited  upon  the  anode  with  the  lead  peroxide.  Silver  peroxide, 
however,  is  very  unstable  in  a  sulphuric-acid  solution  at  the  boiling 
temperature  and  Sand  makes  use  of  this  fact  in  his  method  for 
depositing  lead  in  the  presence  of  silver  (cf.  p.  42).  The  solution 
he  used  contained,  in  about  85  cc.,  0.28  gm.  of  lead  and  0.27  gm. 
of  silver;  it  was  treated  with  10  to  15  cc.  of  concentrated  nitric 
acid  and  was  kept  boiling  during  the  ten  minutes  required  for  the 
electrolysis.  As  outer  electrode  (anode)  for  receiving  the  lead- 
peroxide  deposit,  the  gauze  electrode  shown  on  page  66  was  used 
and  the  inner  electrode  (cathode)  made  300  to  600  revolutions  per 
minute.  The  potential  of  the  current  was  1.6  to  1.7  volts  and 
the  current  strength  was  3  to  4  amperes. 

Before  attempting  to  deposit  the  silver  in  the  solution  freed 
from  lead,  it  is  necessary  to  dissolve  the  small  deposit  of  silver 
that  has  formed  upon  the  cathode  while  the  lead  peroxide  was 
being  precipitated.  The  solution  is  then  transformed  into  an 
acetate  solution  and  the  silver  determined. 

Separation  of  Silver  from  Bismuth. 

Bismuth  is  near  copper  in  the  potential  series  of  the  metals. 
Thus  H.  Freudenberg  *  succeeded,  in  his  analyses  based  upon 
graded  potentials,  in  effecting  a  separation  of  silver  and  bismuth 
in  much  the  same  manner  as  in  the  case  of  silver  and  copper. 
The  best  conditions  for  the  separation  of  silver  and  copper,  how- 
ever, are  those  of  Kiister  and  v.  Steinwehr  (p.  225),  and  these 
should  be  followed  here. 

Rapid  Separation  of  Silver  from  Bismuth. 

This  can  be  effected  in  the  same  way  as  in  Sand's  method  for 
separating  silver  from  copper  (p.  227). 

Separation  of  Silver  from  Platinum. 

According  to  L.  G.  Kollock,|  a  solution  containing  0.2  gm.  of 
each  metal  and  1.25  gms.  of  pure  potassium  cyanide  in  about 
125  cc.  may  be  electrolyzed  at  70°  with  a  current  of  NDioo  = 

*  Z.  phys.  Chem.,  12,  108  (1893).      f  J-  Am.  Chem.  Soc.,  21,  911  (1899). 


SEPARATION  OF  SILVER  FROM  ZINC  249 

0.04  ampere  at  2.5  volts.     In  about  3  hours  all  the  silver  will  be 
deposited. 

For  the  rapid  separation,  the  electrolysis  may  be  carried  out  in 
the  manner  described  for  separating  silver  from  copper  (p.  227). 
About  0.12  gm.  silver  is  deposited  in  20  minutes  with  a  current 
of  3  volts  in  an  electrolyte  containing  1.5  gms.  potassium  cyanide. 
The  current  strength  is  0.25  ampere  at  the  start  but  falls  to  0.05 
ampere  (Julia  Langness). 

Separation  of  Silver  from  Selenium. 

Inasmuch  as  silver  selenite  requires  for  its  solution  more  nitric 
acid  than  should  be  present  in  a  solution  from  which  a  satisfactory 
deposit  of  silver  is  to  be  obtained,  J.  Meyer,*  who  determined  the 
atomic  weight  of  selenium  in  this  way,  used  a  potassium-cyanide 
solution  as  electrolyte.  The  silver  selenite  is  dissolved  in  100  cc. 
water,  some  potassium  cyanide  is  added,  and,  after  heating  to  60° 
or  70°,  the  solution  is  electrolyzed  with  a  potential  of  2.25  volts 
at  the  start.  Toward  the  last  the  electromotive  force  should  be 
increased  to  3.65  volts.  The  above  temperature  is  maintained 
throughout  the  6  hours  required  for  the  complete  deposition  of 
the  silver.  The  voltage  may  be  regulated  with,  the  thermopile 
or  by  means  of  the  arrangement  described  on  page  212.  A  plat- 
inum dish  and  disk  anode  are  suitable  electrodes. 

No  method  is  known  for  the  electrolytic  determination  of 
selenium.  The  potassium-cyanide  solution  from  which  the  silver 
has  been  deposited  is  made  slightly  acid  with  hydrochloric  acid, 
1  or  2  gms.  of  hydrazine  sulphate  f  are  added  and  the  solution  is 
heated  upon  the  water  bath  until  the  precipitated  selenium  is 
changed  into  the  black  modification,  and  the  supernatant  solu- 
tion is  clear.  The  selenium  is  filtered  upon  an  asbestos  filter 
(Gooch  crucible),  washed,  and  dried  at  100°  to  110°  before  weighing. 

Separation  of  Silver  from  Zinc. 

After  E.  F.  Smith  and  Spencer  had  found  that  this  separation 
was  accomplished  much  more  quickly  in  a  hot  solution  containing 
potassium  cyanide  than  in  a  similar  solution  at  the  ordinary  tem- 
perature, Smith  and  Wallace  t  studied  the  conditions  more  closely 
but  paid  no  attention  to  the  voltage  of  the  current  used.  In  this 

*  Z.  anorgan.  Chem.,  31,  391  (1902).       f  P.  Jannasch.,  Ber.,  31,  2393  (1898). 
t  Z.  Elektrochem.,  2,  312  (1895). 


250          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

respect  the  work  was  perfected  by  M.  Heidenreich  in  the  Aachen 
laboratory.  Heidenreich  found  that  the  separation  took  place 
best  at  a  temperature  of  60°  to  70°  with  an  electromotive  force 
of  1.9  to  2  volts.  The  solution  should  contain  0.2  to  0.25  gm. 
silver,  0.16  gm.  zinc  and  2  to  2.5  gms.  of  potassium  cyanide.  As 
electrodes  a  roughened  platinum  dish  and  disk  anode  may  be  used. 
The  current  strength  may  run  from  0.05  to  0.02  ampere.  The 
determination  of  the  silver  will  require  about  6|  hours. 

If  only  1  gm.  of  potassium  cyanide  is  added  to  a  solution  con- 
taining about  0.1  gm.  of  each  metal,  the  electrolysis  may  be 
carried  out,  according  to  L.  G.  Kollock,*  in  3  hours  with  a  current 
of  NDioo  =  0.32  to  0.38  ampere  and  a  potential  of  2.6  volts. 

Rapid  Separation  of  Silver  from  Zinc. 

Julia  Langness  electrolyzed  a  solution  containing  0.12  gm.  silver 
and  2.5  gms.  of  potassium  cyanide  with  a  current  of  0.35*  ampere 
and  potential  of  3  volts  at  the  start;  toward  the  end  the  current 
strength  dropped  to  0.08  ampere.  The  silver  was  all  deposited 
at  the  end  of  20  minutes.  The  other  conditions  were  the  same 
as  given  for  the  separation  of  copper  from  silver  (p.  227). 

MERCURY. 

Separation  of  Mercury  from  Aluminium. 

The  method  is  the  same  as  that  described  for  the  deposition  of 
mercury  in  nitric-acid  solution  (p.  135). 

Separation  of  Mercury  from  Antimony,  Arsenic  and  Tin. 

In  an  ammoniacal  tartrate  solution  the  mercury  may  be  de- 
posited in  the  presence  of  one  or  all  of  these  other  metals.  A 
solution  containing  about  0.1  gm.  of  each  metal  is  treated  with 
8  gms.  tartaric  acid  and  30  cc.  of  10  per  cent  ammonia,  diluted  to 
175  cc.  and  electrolyzed  at  60°  with  a  current  of  NDioo  =  0.05 
ampere  at  1.7  volts.  The  mercury  is  deposited  in  6  hours. 

If  only  one  metal  other  than  mercury  is  present,  the  addition  of 
5  gms.  of  tartaric  acid  and  15  to  30  cc.  of  ammonia  suffices  (S.  C. 
Schmucker).f 

*  J.  Am.  Chem.  Soc.,  21,  911  (1899). 
t  Ibid.,  16,  204  (1893). 


SEPARATION  OF  MERCURY  FROM  SELENIUM          251 

Separation  of  Mercury  from  Alkaline  Earths,  Magnesium 
and  the  Alkalies. 

The  same  conditions  hold  as  for  the  electrolysis  of  mercury  from 
a  nitric-acid  solution. 

Separation  of  Mercury  from  Cadmium,  Cobalt,  Nickel 
and  Iron. 

This  separation  also  is  conducted  in  a  nitric-acid  solution  in  the 
same  manner  as  in  the  separation  of  mercury  from  aluminium. 

In  Potassium-cyanide  Solution  mercury  can  be  separated  from 
cadmium.  To  a  solution  containing  about  0.12  gm.  mercury  and 
0.22  gm.  of  cadmium,  2.5  gms.  of  potassium  cyanide  are  added, 
and  enough  water  to  make  the  total  volume  125  cc.  The  electroly- 
sis is  conducted  with  a  current  of  NDioo  =0.18  ampere  and  poten- 
tial 1.7  volts.  At  the  laboratory  temperature  the  time  required 
is  about  7  hours. 

Separation  of  Mercury  from  Manganese. 

In  a  sulphuric-acid  solution  the  mercury  is  obtained  as  metal 
upon  the  cathode  and  the  manganese  as  dioxide  upon  the  anode 
(dish).  The  latter  deposit,  however,  does  not  always  adhere 
well,  particularly  when  more  than  0.06  gm.  of  manganese  is 
present.  Moreover,  much  mercury  cannot  be  determined  if  a 
disk  is  used  as  cathode;  it  has  a  relatively  small  surface  upon 
which  but  little  mercury  can  be  held  without  its  dropping  off. 
More  mercury  can  be  determined  if  a  platinum  gauze  cathode  is 
used. 

According  to  B.  Neumann,  the  solution  is  prepared  for  elec- 
trolysis by  adding  10  drops  of  concentrated  sulphuric  acid.  The 
current  strength  is  NDioo  =  0.4  to  0.6  ampere  at  4  volts. 

Separation  of  Mercury  from  Selenium. 
E.  F.  Smith  *  effected  this  separation  in  a  potassium-cyanide 
solution  (cf.  p.  136)  containing  about  0.13  gm.  of  mercury,  0.25 
gm.  of  sodium  selenate  and  1  gm.  potassium  cyanide.  The  volume 
of  the  solution  was  150  cc.,  the  temperature  60°  f  and  the  mer- 
cury was  deposited  in  6  hours  by  a  current  of  0.03  ampere  at 
3  volts.  Concerning  the  determination  of  the  selenium,  see  page 
249. 

*  J.  Am.  Chem.  Soc.,  26,  894  (1903). 

t  Regarding  the  possibility  of  some  loss  of  mercury,  see  page  135. 


252  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Separation  of  Mercury  from  Tellurium. 

Smith  succeeded  in  accomplishing  this  separation  in  a  potassium- 
cyanide  solution  (see  the  preceding  paragraph),  but  not  in  a  nitric- 
acid  solution.  The  conditions  were:  about  0.13  gm.  of  mercury, 
0.25  gm.  of  sodium  tellurate,  3  cc.  of  sulphuric  acid  (sp.  gr.  1.43), 
total  volume  150  cc.,  temperature  60°,*  NDi00  =  0.04  to  0.05 
ampere,  potential  2  to  2.25  volts,  time  5  hours. 

Separation  of  Mercury  from  Zinc. 

According  to  Kollock  the  following  conditions  proved  satisfac- 
tory: To  a  solution  containing  0.12  gm.  of  mercury  as  mercuric 
chloride  and  0.1  gm.  of  zinc  as  zinc  sulphate,  2  gms.  of  potassium 
cyanide  were  added,  and  the  solution  having  a  volume  of  125  cc. 
was  electrolyzed  at  50°  with  a  current  of  NDioo  =  0.03  ampere 
and  a  potential  difference  of  2.9  volts.  The  mercury  was  com- 
pletely precipitated  in  4  hours. 

Separation  of  Mercury  from  Bismuth. 

This  can  be  accomplished,  according  to  Sand,  by  the  method 
recommended  for  the  separation  of  mercury  from  copper  (p.  230). 

GOLD. 

Separation  of  Gold  from  Platinum. 

The  solution  of  the  two  metals  is  treated  with  1.5  gms.  of 
potassium  cyanide,  diluted  to  about  350  cc.  and  the  gold  deposited 
at  70°  with  a  current  of  NDioo  =  0.01  ampere  at  2.7  volts.  In 
3  hours  about  0.15  gm.  of  gold  may  be  deposited  in  the  presence 
of  0.1  gm.  of  platinum.  (L.  G.  Kollock.)  f 

Rapid  Separation  of  Gold  from  Platinum. 
Julia  Langness  J  effected  a  successful  separation  under  the  fol- 
lowing conditions.  The  solution  of  the  chlorides,  containing  0.05 
to  0.1  gm.  of  gold  and  0.04  to  0.1  gm.  of  platinum,  was  treated  with 
2  gms.  potassium  cyanide,  diluted  to  125  cc.  and  electrolyzed  at 
the  boiling  temperature  with  2.5  amperes  at  6  volts.  The  spiral 
anode  made  500  to  600  revolutions  per  minute  and  the  analysis 
required  from  15  to  20  minutes. 

*  Cf.  footnote,  p.  251. 

t  J.  Am.  Chem.  Soc.,  21,  923  (1899). 

I  Ibid.,  29,  470  (1907). 


SEPARATION  OF  ANTIMONY   FROM   TIN  253 

Separation  of  Gold  from  Palladium. 

The  gold  is  deposited  under  similar  conditions  as  when  platinum 
is  present.  Potassium  cyanide  2  gms.,  volume  150  cc.,  temper- 
ature 65°,  NDioo  =  0.03  to  0.04  ampere,  2.5  volts.  For  the  deposi- 
tion of  0.13  gm.  of  gold,  5  hours  are  required. 

Rapid  Separation  of  Gold  from  Palladium. 

The  conditions  are  similar  to  the  rapid  method  for  separating 
gold  from  platinum.  Potassium  cyanide  1  gm.,  potential  6  volts, 
and  current  strength  2  amperes.  The  gold  is  deposited  in  10  to 
30  minutes. 

PLATINUM. 
Separation  of  Platinum  from  Iridium. 

As  stated  on  page  141,  a  dense  deposit  of  platinum  may  be 
obtained  with  the  aid  of  a  current  of  NDioo  =  0.05  ampere  at 
1.2  volts  potential.  Under  these  conditions,  all  the  iridium  will 
remain  in  solution. 

ANTIMONY. 
Separation  of  Antimony  from  Tin. 

The  quantitative  separation  of  antimony  from  tin  offers  consider- 
able difficulty  according  to  the  usual  methods  of  gravimetric 
analysis  but  the  electrolytic  separation  is  simple  as  well  as  accu- 
rate. The  quantitative  deposition  of  the  antimony  takes  place  in 
a  concentrated  solution  of  pure  sodium  monosulphide  to  which  a 
certain  amount  of  pure  sodium  hydroxide  has  been  added. 

Sufficiently  pure  commercial  sodium  monosulphide  (free  from 
antimony  and  iron)  can  now  be  purchased.  The  sodium  hydrox- 
ide must  likewise  be  pure  and  the  product  prepared  from  metallic 
sodium  is  to  be  recommended.  Although  the  addition  of  sodium 
hydroxide  is  unnecessary  in  the  absence  of  tin  (cf.  p.  157),  in  this 
case  it  is  required  to  react  with  any  sodium-hydrogen  sulphide 
that  may  be  present.  This  salt  tends  to  prolong  the  time  required 
for  the  complete  deposition  of  the  antimony  and  also  favors  the 
deposition  of  some  tin  with  the  antimony.  Some  sodium-hydro- 
gen sulphide,  NaSH,  may  be  present  in  the  sodium  mono- 
sulphide  or  it  may  be  formed  from  the  latter  as  the  result  of 
hydrolysis: 

Na2S  +  H20  <=>  NaSH  4-  NaOH. 


254  QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 

In  accordance  with  the  mass-action  principle,  the  addition  of 
sodium  hydroxide  prevents  the  hydrolysis. 

To  reduce  polysulphides,  shown  by  a  yellow-colored  solution 
to  be  present,  and  to  prevent  their  formation  during  the  progress 
of  the  electrolysis,  it  is  necessary  to  add  some  potassium  cyanide 
(cf.  p.  156).  As  a  result  of  the  absence  of  polysulphides,  the 
reducing  action  at  the  cathode  is  more  energetic  than  it  would 
be  otherwise,  for  a  part  of  the  current  would  be  used  for  the  reduc- 
tion of  the  polysulphides.  When  polysulphides  are  absent  there 
is  danger  of  some  of  the  current  being  used  for  the  liberation  of 
hydrogen  at  the  cathode  or  for  the  deposition  of  some  tin.  Since 
in  order  to  accomplish  the  complete  deposition  of  the  antimony 
the  potential  of  the  bath  cannot  be  lowered  below  0.8  volt,  it  is 
necessary  to  keep  the  temperature  of  the  bath  close  to  30°  which 
has  been  found  experimentally  to  be  the  most  favorable  tempera- 
ture. In  this  case  the  potential  may  be  as  high  as  1.1  volts. 

The  experiments  of  A.  Fischer  *  at  Aachen  have  established  the 
following  conditions:  The  concentrated,  aqueous  solution  of  the 
salts,  or  the  solid  salts  or  sulphides,  containing  about  0.3  gm.  anti- 
mony and  from  0.3  to  0.5  gm.  of  tin,  is  treated  with  a  solution 
of  sodium  monosulphide  which  is  saturated  with  the  salt  at  30°, 
with  5  to  15  cc.  of  a  30  per  cent  potassium-cyanide  f  solution,  and 
with  a  concentrated  solution  of  about  2  gms.  of  sodium  hydroxide. 
Then,  if  necessary,  enough  more  of  the  sodium-sulphide  solution 
is  added  to  make  the  total  volume  110  to  120  cc.  The  success  of 
the  separation  depends  largely  upon  the  use  of  a  properly  pre- 
pared saturated  sodium-sulphide  solution.  The  current  is  ad- 
justed to  a  potential  of  1.0  to  1.1  volts  and  the  current  strength  is 
then  0.35  to  0.64  ampere.  The  quantity  of  potassium-cyanide 
solution  stated  above  is  regulated  according  to  the  current  strength, 
more  being  added  with  a  strong  current  than  with  a  weak  one. 
The  voltage  and  temperature  (30°)  are  kept  constant  during  the 
entire  operation.  After  7  or  8  hours  the  deposition  of  the  anti- 
mony is  complete  and  the  current  drops  to  between  0.24  and  0.57 

*  Dissertation,  Leipsic,  1904. 

t  Hollard  and  Bertiaux  state  that  the  addition  of  potassium  cyanide  pre- 
vents the  deposition  of  copper.  There  is  little  danger  of  copper  being  present, 
however,  because  the  solubility  of  copper  sulphide  in  alkaline-sulphide  solu- 
tions is  due  to  the  presence  of  sodium-hydrogen  sulphide  or  of  sodium  poly- 
sulphide  and  these  compounds  are  absent  in  the  solution  used  here. 


DETERMINATION  OF  TIN  AFTER  REMOVAL  OF  ANTIMONY  255 

ampere.     The  method  of  testing  to  see  when  the  analysis  is  fin- 
ished was  described  on  page  158.* 

In  discussing  the  determination  of  antimony  by  itself  (p.  157),  it 
was  stated  that  the  presence  of  alkali  hydroxide  caused  the  results 
to  be  a  liUle  too  high.  Experiments  by  Dr.  Scheen  at  Aachen 
have  shown  that  there  is  little  harm  caused  if  not  more  than  about, 
2  gms.  of  pure  sodium  hydroxide  are  used.  For  the  most  accurate 
results,  however,  it  is  advisable  to  dissolve  the  deposit  in  alkali 
polysulphide  solution,  to  add  the  requisite  amount  of  potassium 
cyanide,  and  to  repeat  the  electrolysis. 


Determination  of  Tin  after  the  Removal  of  Antimony. 

It  was  indicated  on  page  160  that  the  deposition  of  tin  from 
the  solution  of  its  thio  salt  has  no  advantages  over  the  electrolysis 
of  an  oxalate  solution.  This  is  especially  true  in  the  case  at  hand. 
The  complete  deposition  of  tin  is  possible  only  in  a  solution  of 
ammonium  sulphide  and  does  not  succeed  in  the  presence  of  sodium 
sulphide.  Moreover,  the  presence  of  potassium  cyanide  increases 
the  difficulty  of  preparing  a  suitable  electrolyte,  because  the  cya- 
nide must  be  removed,  or  a  spongy  tin  deposit  will  be  obtained. 

The  simplest  way  to  prepare  the  electrolyte  for  the  tin  deter- 
mination is  to  acidify  with  acetic  acid,  heat  until  all  the  hydro- 
gen sulphide  and  hydrogen  cyanide  have  been  expelled  and  filter 
off  the  precipitated  tin  sulphide.  After  the  precipitate  has  been 
washed  free  from  the  greater  part  of  the  salts  in  solution,  the  filter 
paper,  with  precipitate,  is  spread  out  on  the  bottom  of  a  small 
dish  and  the  sulphide  dissolved  by  heating  with  water  and  20  gms. 
of  oxalic  acid.  The  solution  is  transferred  to  the  electrolyzing 
vessel,  treated  with  10  gms.  of  ammonium  oxalate  and  the  elec- 
trolysis is  conducted  as  described  on  page  160.  Since  a  large 
quantity  of  oxalic  acid  io  already  present,  it  is  not  usually  neces- 
sary to  add  any  more  during  the  electrolysis  (cf.  p.  160). 

Separation  of  Antimony  from  Arsenic. 

The  fact  that  it  is  not  practicable  to  determine  arsenic  electro- 
lytically  was  mentioned  on  page  162.  In  many  cases,  however, 

*  To  test  the  antimony  deposit  for  tin,  it  is  merely  necessary  to  aUow~a 
little  hydrochloric  acid  to  flow  over  the  deposit  and  to  pour  this  acid  into  a 
solution  of  mercuric  chloride;  a  turbidity  will  result  if  tin  is  present  in  the 
deposit. 


256  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

arsenic  is  deposited  upon  the  cathode  in  the  electrolytic  deter- 
mination of  other  metals  and,  to  prevent  such  contamination, 
special  precautions  have  to  be  taken  in  each  individual  case. 
This  is  especially  true  with  regard  to  the  electrolytic  determination 
of  antimony,  and  here,  as  in  other  cases,  a  marked  difference  is 
observed  in  the  behavior  of  arsenic,  dependent  upon  whether  it 
is  present'  in  the  trivalent  or  quinquevalent  condition.  In  an 
alkaline  solution  arsenious  acid  is  oxidized  to  arsenic  acid  by  the 
action  of  the  electric  current.  If,  however,  a  solution  containing 
both  antimony  and  arsenious  acid  is  electrolyzed,  a  mixture  of 
antimony  and  arsenic  is  deposited.  The  action  is  different  if  the 
arsenic  is  present  in  the  solution  as  arsenic  acid;  in  the  presence  of 
free  alkali,  the  antimony  alone  is  precipitated  from  a  concentrated 
sodium-sulphide  solution. 

To  separate  these  two  elements,  therefore,  any  arsenic  present 
as  arsenious  acid  must  be  oxidized  to  arsenic  acid.  Nitric  acid  or 
aqua  regia  should  be  added  to  the  solution,  the  acid  completely 
expelled  by  evaporating  to  dryness  on  a  water  bath  and  the  residue 
treated  with  80  cc.  of  a  solution  of  sodium  sulphide,  saturated  at 
30°.  The  potassium  cyanide  and  sodium  hydroxide  are  added 
and  the  electrolysis  is  conducted  as  in  the  separation  of  antimony 
from  tin  (cf.  p.  254). 

To  determine  the  arsenic,  the  antimony-free  solution  is  acidified 
with  dilute  sulphuric  acid,  heated  on  the  water  bath  to  expel  the 
hydrogen  sulphide  and  hydrogen  cyanide,  filtered,  and  the  precipi- 
tate dissolved  in  hydrochloric  acid  with  the  addition  of  potassium 
chlorate.  This  solution  is  treated  with  ammonia  in  excess,  and 
the  arsenic  acid  precipitated  as  magnesium-ammonium  arsenate 
with  magnesium  mixture. 

The  precipitate  may  be  dried,  at  110°,  on  a  tared  filter  and 
weighed  as  magnesium  ammonium  arsenate,  or  it  may  be  con- 
verted into  magnesium  pyroarsenate  by  careful  ignition  in  a  por- 
celain crucible. 

Separation  of  Antimony,  Tin  and  Arsenic. 

Since  arsenic  cannot  be  separated  from  tin  electrolytically,  it 
is  necessary  to  determine  the  arsenic  by  the  ordinary  analytical 
methods  before  attempting  to  determine  the  tin  by  electrolysis. 
One  of  the  best  methods  for  separating  arsenic  from  tin  is  the  dis- 
tillation of  the  arsenic  trichloride  from  a  solution  containing  a 


SEPARATION  OF  ANTIMONY,   TIN  AND  ARSENIC       257 

reducing  agent.  The  arsenic  may  be  expelled  first,  and  in  this 
way  separated  from  both  antimony  and  tin;  or  the  antimony  may 
be  determined  electrolytically  and  the  distillation  accomplished 
after  the  removal  of  the  antimony.  The  second  method  requires 
that  the  arsenic  should  be  in  the  quinquevalent  conditions  (see 
preceding  page)  and  thus,  in  most  cases,  a  preliminary  oxidation 
is  necessary.  In  the  subsequent  removal  of  the  arsenic  by  distil- 
lation, it  is  necessary  to  convert  the  arsenic  wholly  into  the  triva- 
lent  condition.  When  the  first  method  is  employed,  i.e.,  when  the 
arsenic  is  distilled  from  a  solution  containing  both  tin  and  anti- 
mony, it  is  immaterial  what  the  condition  of  the  arsenic  is  at  the 
start,  as  enough  reducing  agent  is  added  in  all  cases  to  effect 
the  complete  reduction.  Moreover,  a  further  advantage  of  this 
method  lies  in  the  fact  that  if  hydrogen  sulphide  is  used  as  the 
reducing  agent,  as  recommended  by  Piloty  and  Stock,  no  foreign 
solid  need  be  added  to  the  solution.  Formerly  ferrous  chloride 
was  used  to  reduce  the  arsenate.  It  was  then  necessary  to  pre- 
cipitate the  tin,  or  the  antimony  and  tin,  with  hydrogen  sulphide 
and  to  transform  the  precipitated  sulphides  into  the  soluble  thio 
salts  before  going  on  with  the  electrolysis. 

The  method  for  distilling  arsenic  trichloride  is  discussed  in  many 
textbooks  of  quantitative  analysis  and  will  not  be  considered  in 
detail  here.*  The  solution  remaining  in  the  flask  after  the  distil- 
lation is  boiled  to  expel  hydrogen  sulphide,  neutralized  with 
sodium  hydroxide,  and  treated  with  sodium  monosulphide  solution, 
potassium  cyanide  and  sodium  hydroxide  as  described  on  page  254. 

If  the  arsenic,  antimony  and  tin  are  present  at  the  start  in  the 
form  of  sulphides,  they  are  dissolved  by  warming  with  concentrated 
hydrochloric  acid  and  potassium  chlorate;  the  excess  of  chlorine 
is  expelled,  and  the  solution  rinsed  into  the  distillation  flask  with 
concentrated  hydrochloric  acid. 

Separation  of  Antimony  from  Bismuth. 

These  two  metals  may  be  separated  in  much  the  same  way  as 
copper  and  antimony  (p.  236).  S.  C.  Schmuckerf  treats  the  solu- 
tion with  5  gms.  of  tartaric  acid  and  15  cc.  of  ammonia.  After 
diluting  to  175  cc.  the  solution  is  electrolyzed  at  50°  with  a  cur- 
rent of  NDioo=  0.022  ampere  at  1.8  volts.  After  6  hours  all  the 
bismuth  is  deposited. 

*  Cf.  Treadwell-Hall,  "  Quantitative  Analysis." 

t  J.  Am.  Chem.  Soc.,  16,  203  (1903). 


258  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

ZINC. 

Separation  of  Zinc  from  Manganese. 

The  zinc  is  deposited  from  a  solution  containing  free  oxalic 
acid  (p.  172),  which  prevents  the  deposition  of  any  manganese 
dioxide  upon  the  anode. 

E.  J.  Riederer  *  deposits  the  zinc  in  a  lactic-acid  solution  under 
the  following  conditions.  As  cathode  a  silvered  platinum  dish  is 
used  and  to  obtain  an  even  deposit  it  is  necessary  to  keep  the 
solution  well  stirred;  a  rotating  anode  is  placed  0.5  cm.  from  the 
cathode.  The  electrolyte  contains  about  0.11  gm.  of  zinc  as  sul- 
phate in  230  cc.  (nitrates  of  chlorides  should  not  be  present)  5  gms. 
of  ammonium  lactate,  0.75  gm.  of  lactic  acid  and  2  gms.  of  ammo- 
nium sulphate.  The  temperature  should  lie  between  15°  and  28°. 
The  current  density,  NDioo  =  0.2  to  0.24  ampere  and  the  poten- 
tial, about  3.8  volts.  The  deposition  of  the  zinc  requires  from 
4  to  5J  hours.  The  manganese  content  may  lie  between  0.03 
and  0.35  gm.  During  the  electrolysis  the  color  of  permanganate 
formed  is  darker  in  proportion  to  the  quantity  of  manganese 
present.  The  electrolysis  would  take  too  long  if  carried  out 
below  15°,  and  above  28°  a  crystalline  or  spongy  deposit  of  zinc 
will  be  obtained;  this  is  also  true  if  the  current  density  is  over 
0.3  ampere. 

In  formic-acid  solution,  G.  P.  Scholl  f  carries  out  the  deposition 
of  zinc  in  the  presence  of  manganese  as  follows:  To  the  solution, 
containing  about  0.1  gm.  of  zinc  as  sulphate,  10  cc.  of  formic  acid 
(sp.  gr.  1.06)  and  5  cc.  of  ammonium-formate  solution  (obtained 
by  neutralizing  formic  acid  of  the  above  strength  with  strong  am- 
monia) are  added  and  the  electrolysis  is  conducted  with  a  current 
of  NDioo  =  1  ampere  and  a  potential  of  5.4  volts.  A  roughened 
platinum  dish  is  used  as  cathode  with  the  sieve  anode  shown  on 
page  57.  The  electrolysis  requires  about  11  hours. 

Separation  of  Zinc  from  Aluminium. 

The  separation  is  accomplished  by  depositing  the  zinc  from  an 
oxalate  solution.  Too  high  a  temperature  must  be  avoided  for 
the  reasons  stated  on  page  269. 

*  J.  Am.  Chem.  Soc.,  21,  789  (1899). 
1 1bid.,  25,  1055  (1903). 


SEPARATION  OF  CADMIUM  259 

Separation  of  Zinc  from  Lead. 

The  lead  is  deposited  in  nitric-acid  solution  as  peroxide  (p.  194), 
and  then,  after  neutralizing  with  caustic-potash  solution,  the  zinc 
is  determined  according  to  page  184. 

Separation  of  Zinc  from  Bismuth. 

When  bismuth  is  deposited  from  a  nitric-acid  solution,  the  zinc 
remains  dissolved  and  can  be  determined  subsequently  as  de- 
scribed on  page  169. 

CADMIUM. 

Separation   of   Cadmium   from   Aluminium,    Alkaline    Earths, 
Magnesium  and  the  Alkalies. 

The  separation  can  be  effected  in  a  sulphuric-acid  solution  by  the 
methods  described  on  page  174  et  seq.  It  is  best  to  filter  off  any 
insoluble  sulphates  of  the  alkaline  earths. 

Rapid  Separation  of  Cadmium  from  Aluminium. 

In  a  sulphuric-acid  solution,  Ashbrook  separated  0.27  gm.  of 
cadmium  in  10  minutes  from  an  equal  quantity  of  aluminium 
under  the  following  conditions:  Volume  125  cc.  with  2  cc.  of 
concentrated  sulphuric  acid;  solution  heated  to  boiling  before 
electrolyzing;  current  NDioo  =  5  amperes  at  5  volts.  The  de- 
posits were  somewhat  spongy  but  could  be  weighed  without  loss. 
A  platinum  dish  was  used  as  cathode  and  the  spiral  anode  revolved 
about  600  times  in  a  minute. 

Separation  of  Cadmium  from  Antimony. 

According  to  Schmucker,  these  two  elements  may  be  separated 
in  a  strongly  ammoniacal  solution,  as  in  the  separation  of  copper 
from  antimony  on  page  236. 

Separation  of  Cadmium  from  Arsenic. 

In  Ammoniacal  Tartrate  Solution.  According  to  Schmucker, 
this  separation  is  the  same  as  the  separation  of  cadmium  from 
antimony. 


QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 


In 


di 


The 


Solution,     H.    Fmidenberg*   obtained 
free  from  arsenic,  by  using  only  a  slight  excess 
the  arsenic  in  the  quinquevalent 


and  not  letting  the  potential  rise  above  2.6  to  2.7  volts. 


aration  depends  upon  the  fact  that  there  is  a  greater 
to  form  cadmium  cations  than  trhralent  arsenic  cations 
W-p-233). 


by  controlling  the  cathode  potential  with  the  aid  of  his 
auxiliary  electrode,  was  able  to  effect  a  quantitative  separation. 
This  is  due  to  the  fact  that  cadmium  is  not  deposited  until  the 
of  the  auxiliary  electrode  t  is  more  than  1  volt  and 
tins  voltage  the  bismuth  can  be  deposited. 
The  solution,  containing  about  OL38  gm.  of  bismuth  and  the 
same  quantity  of  cadmium,  was  treated  with  2.5  cc.  of  concen- 
trated nitric  acid,  and  18  gms.  of  tartaric  add,  heated  tb  80°  and 
the  potential  of  the  auxiliary  electrode  adjusted  to  0.43  volt. 
The  potential  between  the  electrodes  was  then  about  1.7  volts 
and  the  initial  current  3  amperes.  The  potential  of  the  auxiliary 
electrode  was  gradually  allowed  to  rise  to  0.53  volt.  At  the  end 
of  10  minutes  all  the  bismuth  was  deposited  and  the  current  had 
sank  to  Ol2  ampere. 

To  *lqnttffl  the  c»Ammmt  the  solution  was  made  *nr*Ktu»  with 
17  gm&  of  soifinm  hydroxide  and  the  cold  solution  electrohxed 
with  a  current  of  2  amperes  at  a  potential  of  2.7  volts.  This  de~ 
18 


In  Sulpkwic-aeid  Solution,  EL  Freudenberg}  succeeded  in  de- 
positing the  cadmium  under  the  following  conditions.  The  solu- 
tion contained  Ol2  gm.  cadmium,  3  to  4  cc.  of  concentrated  solu- 
tion  of  ammonium  sulphate,  and  2  to  3  cc.  of  dihrte  sulphuric  add. 
The  maximum  potential  between  the  electrodes  was  2  A  to  2.9 
volts. 

In  a  Potasaumrcyanide  Solution,  the  same  author  deposited  the 

*Z.pfc^dieiiL,li,122(im). 

i  if  and  hoe  and  at  < 
It  i§  •**  aa  accurate  rtatement:  </.  p.  42. 


'ARATTO.V   Of    CADMIUM   PROM    fkO.V 

in  the  presence  of  an  excess  of  potassium  cyanide  with 
potential  of  2,6  to  2,7  rohs,* 


In  8ulpkuH*aeid  Mutism,  the  conditions  ate  the 


,t 
the  sulphates  of  the  two  metafe  are  dissolved  m  100  ee,  of  water 


2  to  3  gms,  of  poliissiiiiii  cyanide  are  added.  The 
is  then  wanned  until  perfectly  dear,  and  if  it  takes  too 
long  for  the  solution  to  assume  the  yellow  color  of  potassium  ferro- 
cyanide,  a  few  drops  of  a  solution  of  potassium  hydroxide  are 
added.  The  gohrtioo  k  dfluted  to  200  cc,  and  dectroryzed  with 
a  current  of  ND**  -  O05  to  0,10  ampere,  at  the  room  temper* 
Cadmium  win  be  deported.  It  wwefltockctrolyjc  orer* 
but  the  analyas  may  be  hastened  by  inoeaong  the  current 
to  0.4  ampere, 
If  the  solution  contains  eonoderable  ferric  salt,  a  small  quan- 

~Jt    f  __  *_.     f  ____  f  ____  '_!_.  •-  *?-  -  -.1—  i-  I    '        *!«._,    __  *  ___  ?  -  .  _ 

my  oi  icmc  jjij^irHBtiMiy  remains'  unuMKiKveoj.  in  r.nf;  por/i.-rhiiinri— 
cyanide  solution.  Although  this  precipitate  does  little  harm,  it 
is  better  to  reduce  the  ferric  salt,  before  adding  the  potassium 
cyanide,  by  the  addition  of  sodium  sulphite  to  the  solution  slightly 
»-ir.h  ralpbum  acid, 


method  may  be  used  as  described  for  the  a 


(Ashbrook,  c}.  p.  259),' 
A,  L,Dayison(  succeeded  in 
25  '""MPJpy,  about  0,26  gm.  of  cadmium  in  the 
of  0.25  gm.  of  mm.  The  electrolyte  was  prepared,  as 
described  above,  by  the  addition  of  12  gms,  of  potassium  cyanide 
and  2  gms.  of  sodium  hydroxide,  and,  after  heating  to  boiling,  it 
was  elciftitJjratJ  with  a  curivnt  of  NDioo  =  5 _  amperes  at  5 
A  platinum  dL-h  wa=  n^e<i  a.=  oarho^  and 
to  rerolve  700  times  per  minute. 


Am.  Chem.  Sbe^  JS,  1288  (190i);  A.  L.  Dfcivigon.  7&uf.r  27, 1286  (1905), 
tZ.EfcJUimJhM^*,4OP(18B8). 

J  J.  Am.  Chem.  3oe.T  «T,  1286  (1906), 


262  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Separation  of  Cadmium  from  Lead. 

See  Separation  of  Lead  from  Other  Metals,  page  284. 

Separation  of  Cadmium  from  Manganese. 

According  to  E.  F.  Smith  the  simultaneous  deposition  of  cad- 
mium upon  the  cathode  and  of  manganese  dioxide  upon  the  anode 
may  be  effected  under  the  following  conditions.  The  solution  is 
acidified  with  2  to  3  cc.  of  sulphuric  acid  (sp.  gr.  1.09),  diluted  to 
125  cc.  and  electrolyzed  at  65°  with  a  current  of  NDioo  =  0.08 
ampere  at  2.6  volts.  A  roughened  platinum  dish  is  used  as  the 
anode. 

Separation  of  Cadmium  from  Mercury. 

In  Sulphuric-acid  Solution.  To  125  cc.  of  the  solution,  3  cc. 
of  concentrated  sulphuric  acid  are  added  and  the  electrolysis  is 
carried  out  at  65°  with  a  current  of  NDioo  =  0.5  ampere  at  3.5 
volts.  The  deposit  is  washed  before  breaking  the  circuit. 

After  concentrating  the  solution  and  washings  the  cadmium 
may  be  determined  directly  or  after  transformation  into  a  potas^- 
shim-cyanide  solution  (cf.  p.  176). 

In  Potassium-cyanide  Solution.  Kollock  *  describes  an  experi- 
ment in  which  2.5  gms.  of  potassium  cyanide  were  added  to  a  solution 
containing  0.1182  gm.  of  mercury  as  mercuric  chloride  and  0.2  gm. 
of  cadmium  as  sulphate,  and  the  solution  having  a  volume  of  125  cc. 
was  electrolyzed  at  65°  with  a  current  of  NDioo  =  0.018  ampere 
and  a  potential  difference  of  1.7  volts.  The  mercury  was  com- 
pletely precipitated  in  7  hours.  At  lower  temperatures  and  with 
higher  current  density  the  cadmium  is  deposited  with  the  mercury. 

After  the  deposit  of  mercury  had  been  removed,  the  cadmium 
was  determined  with  a  stronger  current  (cf.  p.  176). 

Separation  of  Cadmium  from  Nickel. 

In  Sulphuric-acid  Solution,  the  separation  is  carried  out  as 
described  for  the  separation  of  cadmium  from  manganese. 

The  rapid  separation  is  the  same  as  for  the  separation  of  cad- 
mium from  aluminium. 

In  Potassium-  cyanide  Solution.  Owing  to  the  slight  stability 
of  the  complex  potassium  cadmium  cyanide,  the  separation  of 
*  J.  Am.  Chem.  Soc.,  21,  919  (1899). 


SEPARATION  OF  CADMIUM  FROM   NICKEL  263 

cadmium  from  cobalt  was  accomplished  without  any  difficulty 
(p.  228),  but  the  separation  of  cadmium  from  nickel  was  first 
attempted  in  vain  by  E.  F.  Smith  and  L.  K.  Frankel  as  well  as  by 
H.  Freudenberg.*  This  difference  in  the  behavior  of  nickel  and 
cobalt  is  explained  by  the  well-known  fact  that  the  cobaltocyanide 
anion  is  readily  changed  by  oxidation  into  the  more  stable  cobalti- 
cyanide  anion 

2  [Co(CN)6]  ==+  H2O  +  O  =  2  [Co(CN)6]-  -  +  2  OH~ 

The  nickelocyanide  does  not  experience  the  corresponding  change 
but  tends,  rather,  to  break  down  into  nickel  cations  and  cyanogen 
anions. 

K2  [Ni(CN4)l  <=*  2  K+  +  [Ni(CN)4]- 
[Ni(CN)J—  <=*  Ni(CN)2  +  2  (CN)~ 
2  (CN)~ 


Smith  and  Frankel  attributed  the  fact  that  the  cadmium  deposit 
contained  nickel  to  the  presence  of  nickelous  cations.  These 
authors  finally  succeeded  in  effecting  a  separation  by  adding 
potassium  hydroxide  to  the  solution.  The  alkali  tends  to  pre- 
vent the  breaking  down  of  the  potassium  nickelocyanide.  The 
separation  was  carried  out  in  the  following  manner. 

To  the  solution,  containing  about  0.17  gm.  of  cadmium  and  0.16 
gm.  of  nickel,  3  gms.  of  potassium  cyanide  and  2  gms.  of  potassium 
hydroxide  (or  NaOH)  were  added  and,  after  diluting  to  175°,  the 
solution  was  electrolyzed  at  40°  with  a  current  of  NDioo  =  0.03  to 
0.04  ampere  at  2.25  to  3  volts. 

To  determine  the  nickel  in  the  solution  after  it  has  been  freed 
from  cadmium,  the  cyanide  may  be  decomposed  by  boiling  with 
sulphuric  acid  under  the  hood  and  the  nickel  determined  in  am- 
moniacal  solution  as  described  on  page  185. 

Separation  of  Cadmium  from  Silver. 

The  silver  can  be  deposited  in  nitric-acid  solution  by  the  method 
given  on  page  132,  using  a  maximum  potential  of  1.35  to  1.38 
volts. 

The  nitric-acid  solution  may  then  be  changed  into  an  acetic-acid 
solution  by  adding  sufficient  sodium  acetate  to  combine  with  the 
nitric  acid.  From  this  solution,  heated  to  50°,  the  cadmium  is 

*  Z.  phys.  Chem.,  12,  122  (1893). 


264  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

deposited  by  a  current  of  NDioo  =  0.05  to  0.06  ampere  at  3.5 
volts.  The  washing  is  done  before  the  current  is  turned  off. 

The  silver  may  also  be  deposited  from  a  potassium-cyanide  solu- 
tion by  adding  2  gms.  of  potassium  cyanide  to  the  solution  con- 
taining about  0.15  gm.  of  each  metal.  The  solution  is  diluted  to 
125°  and  electrolyzed  at  about  70°  with  a  current  of  NDioo  =  0.02 
ampere  at  2.1  volts. 

The  potassium-cyanide  solution  may  be  analyzed  for  cadmium 
as  described  on  page  176. 

Separation  of  Cadmium  from  Zinc. 

In  Sulphuric-acid  Solution.  The  values  for  the  decomposition 
potentials  of  these  two  metals  (in  normal  solution:  CdS04  =  2.24 
volts,  ZnSOi  =  2.54  volts)  is  sufficient  to  permit  an  electrolytic 
separation  of  these  two  elements.  Moreover,  on  account  of  the 
overvoltage  of  hydrogen  toward  cadmium,  the  electrolysis* can  be 
carried  out  in  fairly  acid  solution  and  it  is  thus  possible  to  sepa- 
rate cadmium  from  relatively  large  quantities  of  zinc.  According 
to  P.  Denso,*  the  solution  containing  0.1  gm.  of  each  metal  is  made 
double-normal  with  respect  to  sulphuric  acid  (1  per  cent  H2SO4) 
and  electrolyzed  with  gauze  electrodes.  The  solution  has  a  volume 
of  about  100  cc.  and  the  potential  of  the  current  should  not  exceed 
2.6  volts, f  corresponding  to  a  current  strength  of  0.08  ampere. 
The  deposition  of  this  quantity  of  cadmium  is  complete  at  the 
laboratory  temperature  in  6  hours. 

From  a  solution  containing  8  gms.  of  zinc  and  about  0.2  gm. 
of  cadmium,  made  normal  with  sulphuric  acid,  the  cadmium  was 
deposited  quantitatively  overnight  with  a  current  of  2.6  to  2.88 
volts  (0.22  ampere). 

This  separation  in  sulphuric-acid  solution  by  the  use  of  a  graded 
potential  is  desirable,  because  copper,  which  is  found  frequently 
in  the  presence  of  cadmium  and  zinc,  can  be  deposited  from  sul- 
phuric-acid solution  (cf.  p.  116). 

After  the  removal  of  the  cadmium,  the  zinc  may  be  deposited 
in  alkaline  solution  after  neutralization  of  the  free  acid  (cf.  p.  184). 

In  Acetic-acid  Solution.     The  solution  containing  the  two  metals 

*Z.  Elektrochem.,  9,  470  (1903). 

t  The  potential  of  the  bath  must  exceed  somewhat  the  decomposition 
potential  in  order  to  overcome  the  Ohm's  resistance  of  the  electrolyte. 


SEPARATION  OF  CADMIUM   FROM  ZINC  265 

in  about  100  cc.  is  treated  with  3  gms.  of  sodium  acetate  and  a 
few  drops  of  acetic  acid.  The  cadmium  is  deposited  at  about 
70°  with  a  current  of  NDioo  =  0.10  ampere  at  2.2  volts.  The 
deposition  of  0.2  gm.  of  metal  requires  4  hours  (A.  Yver).* 

In  the  laboratory  of  the  Munich  Polytechnic  School  the  fol- 
lowing modification  of  Yver's  method  is  in  use:  To  a  sulphuric- 
acid  solution  of  the  two  metals,  sodium-hydroxide  solution  is  added 
until  a  permanent  precipitate  is  formed,  the  precipitate  is  dis- 
solved by  adding  the  smallest  possible  quantity  of  dilute  sulphuric 
acid,  the  solution  is  diluted  to  about  70  cc.,  and  the  cadmium  is 
precipitated  with  a  current  of  NDioo  =  0.07  ampere.  When  the 
greater  part  of  this  metal  has  been  precipitated,  the  free  sulphuric 
acid  is  neutralized  with  sodium  hydroxide,  3  gms.  of  sodium 
acetate  are  added,  the  solution  is  warmed  to  about  45°  and  elec- 
trolyzed  with  a  current  of  NDioo  =  0.3  ampere  and  a  potential 
difference  of  about  2.4  volts. 

In  Oxalate  Solution.  The  experiments  of  S.  Eliasberg  f  and  of 
A.  Waller  { in  the  Aachen  laboratory  have  shown  that  the  following 
conditions  will  give  favorable  results.  In  the  solution  of  the 
chlorides  at  a  volume  of  about  120  cc.,  8  gms.  of  potassium  oxalate 
and  2  gms.  of  ammonium  oxalate  are  dissolved,  the  solution  heated 
to  80°  or  85°  and  the  cadmium  deposited  with  a  current  of  0.1  to 
0.3  ampere,  keeping  the  potential  at  2.4  volts  or  less. 

The  deposit  is  washed  while  the  current  is  passing  and  in  this 
way  the  zinc  solution  becomes  much  diluted.  It  is  concentrated 
by  evaporation  and  the  zinc  determined,  after  making  acid  with 
tartaric  acid,  according  to  page  173. 

IRON. 
Separation  of  Iron  from  Nickel  and  Cobalt. 

As  mentioned  on  page  182,  the  iron  deposits  obtained  from 
tartaric-  or  citric-acid  solutions  invariably  contain  carbon.  For 
this  reason,  the  methods  based  upon  the  electrolytic  deposition 
of  iron  from  such  solutions  will  not  be  discussed  here. 

G.  Vortmann§  has  found  that  ferric  hydroxide  suspended  in  an 
ammoniacal  solution  is  not  acted  upon  by  a  current  of  1.0  ampere 

*  Bull.  soc.  chim.,  34,  18  (1880). 

t  Z.  anal.  Chem.,  24,  548  (1885). 

j  Z.  Elektrochem.,  4,  241  (1897). 

§  Monatsh.,  14,  536  (1893). 


266          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

or  less  and  that  from  an  ammoniacal  solution  such  metals  as 
cobalt,  nickel  and  copper  can  be  deposited  quantitatively.  To 
convert  the  iron  into  the  condition  in  which  it  has  no  bad  effect, 
the  solution  is  oxidized  by  bromine  water  and  after  the  excess  of 
the  reagent  has  been  removed  by  heating  the  solution,  3  to  6  gms. 
of  ammonium  sulphate  are  added  and  the  iron  is  precipitated  by  a 
liberal  excess  of  ammonia. 

The  objection  that  the  deposits  may  be  injured  by  long  contact 
with  the  ferric-hydroxide  precipitate  is  removed  if  the  wire-gauze 
electrode  shown  on  page  59  is  used  as  cathode  instead  of  a  plat- 
inum dish,  and  the  stirrer  is  given  a  moderate  velocity  (cf.  p.  238). 

With  a  current  density  of  NDioo  =  0.4  to  0.8  ampere,  as  much  as 
0.1  gm.  of  nickel  is  deposited  in  2  or  3  hours.  As  a  rule  the  deposit 
will  contain  traces  of  iron,  and  for  the  most  accurate  results  the 
metal  should  be  dissolved  in  dilute  sulphuric  acid,  with  the  addi- 
tion of  a  little  bromine,  and  the  electrolysis  repeated  in  this  solu- 
tion under  the  same  conditions  as  before. 

To  determine  when  the  electrolysis  is  complete,  a  little  of  the 
clear  ammoniacal  solution  is  tested  with  ammonium  sulphide;  if 
no  brown  coloration  appears,  all  the  cobalt  or  nickel  has  been 
deposited  (cf.  p.  188). 

Separation  of  Iron  from  Zinc. 

If  a  solution  containing  ammonium-iron  oxalate  and  either 
ammonium-zinc  oxalate  or  potassium-zinc  oxalate  is  electrolyzed, 
then,  when  less  than  one  third  as  much  zinc  as  iron  is  present,  the 
two  metals  deposit  together  upon  the  cathode.  If  the  zinc  content 
is  higher,  it  is  noticed  that  zinc  during  the  progress  of  the  electrolysis 
begins  to  dissolve  with  evolution  of  hydrogen  and  at  the  same  time 
a  precipitate  of  ferric  hydroxide  forms.  The  method,  therefore, 
is  limited  in  its  application;  it  is  carried  out  as  described  for  the 
electrolytic  determination  of  iron  (p.  183). 

After  the  total  weight  of  iron  and  zinc  has  been  determined,  the 
deposit  is  dissolved  in  dilute  sulphuric  acid  and  the  iron  titrated 
in  the  platinum  dish  with  potassium-permanganate  solution. 

G.  Vortmann*  deposits  the  zinc  alone,  by  keeping  the  iron  in 
solution  as  potassium  ferrocyanide.  If  necessary  the  iron  is  re- 
duced to  the  ferrous  state  by  treatment  with  sulphurous  acid, 

*  Monatsh.,  14,  549  (1893). 


SEPARATION  OF  IRON  FROM   MANGANESE         267 

then  enough  potassium  cyanide  is  added  to  dissolve  the  precipitate 
that  first  forms,  and  caustic-soda  solution  is  added. 

A  too  great  excess  of  potassium  cyanide  has  an  injurious  effect 
upon  the  deposition  of  the  zinc.  The  electrolysis  is  carried  out 
in  a  copper-  or  silver-  coated  platinum  dish  with  a  current  density 
of  NDioo  =  0.3  to  0.6  ampere.  When  a  portion  of  the  solution 
shows  no  turbidity  on  being  heated  with  a  few  drops  of  ammonium- 
sulphide  solution,  all  the  zinc  will  have  been  deposited. 

Separation  of  Iron  from  Manganese. 

Most  of  the  methods  recommended  for  the  electrolytic  sepa- 
ration of  iron  and  manganese  have  no  practical  value  because 
they  lead  to  the  determination  of  the  iron  rather  than  to  that  of 
the  manganese.  Moreover  the  deposition  of  the  iron  from  an 
oxalic-acid  solution  containing  manganese  gives  rise  to  the  precipi- 
tation of  considerable  ferric  hydroxide  with  the  manganese  dioxide 
at  the  anode.  Thus  the  attempt  has  been  made  to  prevent  the 
formation  of  a  manganese-dioxide  deposit,  or  to  defer  its  forma- 
tion as  long  as  possible. 

The  first  method  proposed  was  to  add  a  large  excess  of  ammo- 
nium oxalate  and  to  electrolyze  at  70°  (Classen).  In  the  hot 
liquid  there  is  a  strong  hydrolysis  of  ammonium  oxalate  so  that 
free  oxalic  acid  is  formed.  This  exerts  a  reducing  effect  upon  the 
manganese  dioxide  and  prevents  the  formation  of  ammonium 
carbonate  which  causes  the  precipitation  of  ferric  hydroxide. 
Good  results  are  obtained,  however,  only  when  the  manganese 
content  of  the  solution  is  not  too  high. 

A.  Hollard  and  L.  Bertiaux  *  carry  out  the  electrolysis  in  an 
ammoniacal-citrate  solution  and  add  sulphurous  acid  as  the  reduc- 
ing agent.  The  deposited  iron  contains  carbon  and  sulphur  (cf. 
p.  182;. 

J.  K6ster,f  while  working  in  the  Aachen  laboratory,  successfully 
used  phosphorous  acid  as  the  reducing  agent.  The  solution  of  the 
double  oxalates  is  prepared  by  the  addition  of  about  10  gms.  of 
ammonium  oxalate  (cf.  p.  182),  and  the  electrolysis  is  conducted  in 
a  roughened  platinum  dish  at  the  ordinary  temperature,  with  a 
current  of  NDioo  =  1.5  to  2  amperes  at  3  to  4  volts.  As  soon  as 
a  deposit  of  manganese  dioxide  begins  to  be  noticed  at  the  anode, 
a  few  cubic  centimeters  of  a  10  per  cent  phosphorous  acid  solution 
*  Bull.  soc.  chim.,  29,  926  (1903).  f  Ber.,  36,  2716  (1903). 


268  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

are  added  and  this  causes  the  dioxide  to  dissolve,  imparting  a  pink 
color  to  the  liquid.  The  addition  of  this  reagent  is  repeated 
from  time  to  time,  but  the  quantity  added  is  kept  as  small  as 
possible  as  it  tends  to  delay  the  deposition  of  the  iron.  After  the 
manganese  has  been  kept  in  solution  for  2  hours,  no  attention  is 
paid  to  the  formation  of  deposit  at  the  anode  as  the  amount  of 
iron  remaining  in  solution  is  too  small  to  be  effected;  thus  no 
more  phosphorous  acid  solution  is  added.  As  a  rule  the  addition 
of  5  cc.  of  the  reagent  suffices  for  an  entire  analysis.  The  de- 
position of  the  iron  requires  about  6  hours.  If,  however,  so 
much  phosphorous  acid  was  added  that  no  manganese  deposit 
is  formed  at  the  anode  (e.g.,  15  cc.  with  0.6  gm.  of  manganese) 
then  the  iron  will  not  be  deposited  until  7  or  8  hours  have  elapsed. 

After  the  iron  is  all  down,  the  dish  is  washed  without  breaking 
the  circuit  and  treated  as  described  on  page  184.  Any  manganese- 
dioxide  precipitate  adhering  to  the  dish  may  be  removed  by  a  soft 
hair  brush. 

In  the  solution  from  which  the  iron  has  been  removed,  the 
manganese  is  determined  as  sulphide. 

Simultaneous  Deposition  of  Iron  and  Manganese  Dioxide. 

Using  the  roughened  platinum  dish  as  anode  and  the  sieve  shown 
on  page  57  as  cathode,  G.  P.  Scholl  *  succeeded  in  electrolyzing 
a  solution  containing  0.1  gm.  of  manganese  as  sulphate  and 
0.1  gm.  of  iron  as  ferric-ammonium  alum;  the  iron  was  deposited 
upon  the  sieve  and  the  manganese  dioxide  upon  the  dish.  To  the 
solution,  5  cc.  of  formic  acid  (sp.  gr.  1.06)  and  10  cc.  of  ammonium- 
acetate  solution  (content  not  specified)  were  added  and  the  solu- 
tion was  electrolyzed  at  the  laboratory  temperature  with  a  current 
of  NDioo  =  1.1  amperes  at  3.9  volts.  After  5  hours  the  elec- 
trolysis was  finished  as  shown  by  raising  the  level  of  the  solution. 
The  deposits  were  washed  without  breaking  the  circuit  and  the 
iron  was  dissolved  in  sulphuric  acid  and  titrated  with  perman- 
ganate. The  direct  weighing  of  the  deposit  would  not  give  as 
accurate  results  because  of  the  presence  of  carbon.  After  dis- 
solving in  sulphuric  acid,  the  hydrocarbons  had  to  be  boiled  off 
and  the  ferric  salt,  formed  during  this  operation,  reduced  by  zinc. 
The  manganese  deposit  was  treated  as  described  on  page  198. 

*  J.  Am.  Chem.  Soc.,  25,  1054  (1903). 


SEPARATION  OF  IRON   FROM   ALUMINIUM  269 

Separation  of  Iron  from  Aluminium. 

When  a  solution  containing  the  above  metals  and  a  great  excess 
of  ammonium  oxalate  is  electrolyzed  in  the  cold,  iron  is  deposited 
on  the  negative  electrode,  while  the  aluminium  remains  in  solution 
as  long  as  the  quantity  of  unchanged  ammonium  oxalate  present 
in  the  solution  is  larger  than  the  quantity  of  ammonium  bicar- 
bonate formed  from  it  by  the  action  of  the  current.  The  current 
must  not  be  allowed  to  pass,  therefore,  longer  than  is  requisite  for 
the  complete  deposition  of  the  iron.  This  is  determined  by  with- 
drawing a  little  of  the  solution  from  time  to  time,  by  means  of  a 
small  capillary  tube,  and  testing  with  an  excess  of  hydrochloric 
acid  and  potassium  thiocyanate  (cf.  p.  183). 

The  process  is  as  follows:  The  aqueous  or  weakly  acid  solution 
(neutralized  with  ammonia  if  necessary)  of  the  sulphates  (the 
chlorides  are  less  suitable)  is  treated  with  about  8  gms.  of  ammo- 
nium oxalate. 

If  the  temperature  of  the  solution  is  not  over  40°,  it  may  be 
submitted  to  electrolysis  at  once,  as  it  cools  sufficiently  while  the 
current  is  passing.  The  current  density  is  NDioo  =  0.5  to  1.0 
ampere  and  the  potential  of  the  current  is  2.75  to  3.8  volts.  About 
0.1  gm.  of  iron  will  be  deposited  in  5  or  6  hours.  Stronger  currents 
and  higher  temperatures  favor  the  precipitation  of  aluminium 
hydroxide. 

If  the  quantity  of  aluminium  present  is  not  greater  than  that 
of  the  iron,  the  method  gives  good  results.  If  a  large  excess  of 
aluminium  is  present,  or  if  the  current  is  allowed  to  act  for  too 
long  a  time,  a  part  of  the  aluminium  is  precipitated  as  hydroxide, 
and  clings  so  closely  to  the  negative  electrode  that  it  is  removed 
with  difficulty. 

In  such  a  case  it  is  necessary,  without  stopping  the  current,  to 
bring  the  aluminium  hydroxide  into  solution  by  adding  oxalic  acid 
little  by  little  to  the  solution  until  there  is  no  more  evolution 
of  carbon  dioxide  and  the  aluminium  precipitate  has  all  dissolved. 
It  is  well  to  pour  the  oxalic-acid  solution  upon  the  watch  glass 
which  covers  the  dish  and  it  can  then  slowly  run  into  the  elec- 
trolyte through  a  perforation  in  the  center  of  the  watch  glass. 
The  electrolysis  must  be  continued  until  all  the  iron  is  deposited. 

To  determine  the  aluminium  in  the  liquid  poured  off  from  the 
iron  deposit,  the  ammonium  oxalate  still  remaining  is  decomposed 
by  the  current  and  the  solution  finally  heated  in  a  porcelain  dish 


270  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

to  expel  the  excess  of  ammonia.  The  precipitate  of  aluminium 
hydroxide  is  filtered  off  and  converted  into  Al2Os  by  ignition  in  a 
crucible.* 

Rapid  Separation  of  Iron  from  Aluminium. 

A.  Fischer  in  the  Aachen  laboratory  used  for  this  separation  a 
platinum  dish  as  cathode  and  a  disk  anode  making  600  revolutions 
per  minute.  About  0.2  gm.  of  iron  was  separated  from  the  same 
quantity  of  aluminium  in  35  minutes.  The  solution  contained 
7  gms.  of  ammonium  oxalate  in  125  cc.  and  was  electrolyzed  at 
70°  with  a  current  of  7  amperes  at  6  to  7  volts. 

To  dissolve  small  quantities  of  aluminium  hydroxide  adhering 
to  the  deposited  iron,  the  dish,  after  the  usual  washing,  was  rinsed 
with  a  dilute  solution  of  potassium  hydroxide,  and  finally  with 
water  again. 

Separation  of  Iron  from  Uranium. 

The  iron  is  deposited  under  the  same  condition  as  in  the  presence 
of  aluminium  [p.  269).  In  this  case  the  formation  of  ammonium 
bicarbonate  has  the  favorable  effect  of  keeping  the  uranium  in 
solution.  The  current,  therefore,  must  not  be  made  strong  enough 
to  heat  the  solution  above  40°,  as  above  this  temperature  the 
bicarbonate  is  decomposed  and  the  ammonium  uranate  precipi- 
tated. 

In  the  solution  freed  from  iron,  the  oxalate  is  decomposed  by 
continuing  the  electrolysis,  and  then  the  solution  is  heated  to 
decompose  the  ammonium  bicarbonate,  whereupon  the  ammonium 
uranate  precipitates.  The  precipitate  is  in  such  a  finely  divided 
condition  that  it  is  hard  to  filter;  it  is,  therefore,  dissolved  by  heat- 
ing with  nitric  acid  and  ammonium  uranate  is  reprecipitated  by 
the  addition  of  ammonia.  It  is  ignited  and  weighed  as  U3O8. 

Separation  of  Iron  from  Chromium. 

A  solution  containing  a  chromic  salt  is  converted  into  ammo- 
nium-chromium oxalate  after  an  excess  of  ammonium  oxalate  has 
been  added.  If  such  a  solution  is  subjected  to  electrolysis,  all  the 
chromium  is  converted  into  chromate.  If  iron  is  also  present  in 
the  solution,  it  is  deposited  as  metal  upon  the  cathode,  and  the 

*  The  fact  that  iron  in  approximately  neutral  solution  is  readily  taken  up 
by  the  mercury  cathode  has  been  utilized  by  E.  F.  Smith  for  the  quantitative 
determination  of  iron  in  the  presence  of  aluminium. 


RAPID  SEPARATION  OF  IRON  FROM   CHROMIUM         271 

deposit,  in  this  case,  is  characterized  by  a  particularly  brilliant 
luster.  The  solution  of  the  two  metals  is  treated  with  8  gms.  of 
ammonium  oxalate,  diluted  to  about  120  cc.,  and  the  electrolysis 
is  conducted  at  65°  with  a  current  of  NDioo  =  1.5  to  2  amperes  at 
3  to  3.4  volts  potential.  The  deposition  of  about  0.11  gm.  iron 
requires  from  4  to  5  hours. 

After  the  iron  is  all  deposited,  the  liquid  is  poured  off,  boiled  to 
decompose  the  ammonium  bicarbonate,  and  the  chromium  is  re- 
duced to  chromic  salt  again  by  boiling  with  hydrochloric  acid  and 
alcohol.  From  the  resulting  green  solution,  chromic  hydroxide  is 
precipitated  with  ammonia,  taking  the  usual  precautions,  and  de- 
termined as  Cr203. 

Rapid  Separation  of  Iron  from  Chromium. 

The  deposition  of  0.2  gm.  of  iron  in  the  presence  of  an  equal  quan- 
tity of  chromium  was  accomplished  by  A.  Fischer  in  45  minutes 
under  the  following  conditions.  Volume  of  solution  100  to  120  cc., 
addition  of  7.5  gms.  of  ammonium  oxalate,  temperature  75°  to 
80°,  current  strength  6.9  to  7.1  amperes,  potential  5.6  to  7  volts. 
As  cathode  a  roughened  platinum  dish  is  used  and  as  anode  the 
corrugated  foil  electrode  (p.  61).  This  form  of  anode  is^  chosen 
for  the  following  reason.  During  the  electrolysis  there  is  consider- 
able foaming  and  spattering  of  tiny  drops  of  the  electrolyte.  If 
an  ordinary  disk  anode  is  used  and  the  dish  is  covered  with  the 
ordinary  type  of  watch  glass,  then  the  bottom  of  the  latter  will 
become  covered  with  the  spattered  solution  from  which  ferric 
hydroxide  is  deposited  and  dissolved  only  with  difficulty  by  oxalic 
acid.  This  objection  is  overcome  by  using  the  corrugated  elec- 
trode and  a  watch  glass  of  unusually  deep  curve.  The  anode  is 
made  to  revolve  only  moderately  fast  and  the  surface  of  the  liquid 
is  kept  in  constant  contact  with  the  bottom  of  the  watch  glass, 
thus  preventing  the  deposition  of  the  ferric  hydroxide  upon  it. 
After  setting  the  anode  in  motion,  therefore,  enough  water  is 
added  to  the  solution  to  make  it  touch  the  glass. 

Separation  of  Iron,  Aluminium  and  Chromium. 

The  method  is  the  same  as  for  the  separation  of  iron  from  alumin- 
ium. The  temperature  there  prescribed  is  sufficient  for  the  oxida- 
tion of  the  chromium  and  must  be  maintained  on  account  of  the 
likelihood  of  some  aluminium  being  present. 


272  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

To  determine  the  aluminium  and  chromium,  the  electrolysis  is 
continued  after  the  removal  of  the  iron  until  the  ammonium 
oxalate  is  wholly  decomposed  (cf.  p.  209),  the  free  ammonia  is  ex- 
pelled by  boiling,  and  the  aluminium  hydroxide  is  filtered  off.  In 
the  filtrate,  the  chromium  is  determined  as  outlined  on  page  271. 

Separation  of  Iron,  Chromium  and  Uranium.* 

The  separation  is  accomplished  by  the  precipitation  of  iron  as 
metal  from  the  double-oxalate  solution,  and  the  oxidation  of 
chromium  to  chromic  acid  by  the  current;  the  uranium  remains 
dissolved  in  the  ammonium-bicarbonate  solution  (p.  270).  To 
accomplish  the  quantitative  separation  of  chromium  from  uranium, 
the  electrolysis  must  be  continued  till  the  oxalic  acid  is  completely 
oxidized.  The  solution  is  then  boiled  to  decompose  the  ammo- 
nium bicarbonate,  and  allowed  to  stand  6  hours  so  that  the  pre- 
cipitate will  assume  a  condition  suitable  for  filtratioii.  The 
chromium  is  determined,  as  above,  in  the  nitrate  from  the  uranium 
(p.  271). 

Separation  of  Iron  from  Beryllium,  f 

The  separation  of  these  two  metals  offers  no  difficulties  what- 
ever if  the  soluble  double  salts  with  ammonium  oxalate  are  pre- 
pared, and  if  care  is  taken  to  have  an  excess  of  ammonium  oxalate 
present  as  in  the  separation  of  iron  from  aluminium  (p.  271). 
The  beryllium  hydroxide  behaves  toward  ammonium  bicarbonate 
as  uranium  hydroxide  does;  to  keep  it  in  solution  strong  currents 
and  rise  in  temperature  must  be  avoided. 

The  determination  of  beryllium  in  the  solution  freed  from  iron 
is  very  simple:  after  all  the  oxalate  has  been  destroyed  by  the 
action  of  the  electric  current,  the  solution  is  boiled  to  decompose 
the  ammonium  carbonate  and  the  heating  is  continued  until  the 
liquid  smells  only  faintly  of  ammonia.  The  beryllium  hydroxide 
is  then  filtered  off,  washed  with  hot  water  and  converted  into  BeO 
by  ignition  in  a  platinum  crucible. 

In  a  sulphuric-acid  solution,  R.  E.  Myers  j:  has  effected  the 
separation  of  iron  from  beryllium  with  the  aid  of  the  mercury 
cathode  (p.  205)  under  the  following  conditions.  The  solution 

*  Classen,  Ber.,  14,  2771  (1881);  17,  2483  (1884). 

t  Classen,  Ber.,  14,  2771  (1881). 

J  J.  Am.  Chem.  Soc .,  26,  1134  (1904). 


SEPARATION  OF  IRON   FROM   ALUMINIUM,  ETC.         273 

contained  0.02  to  0.2  gm.  of  iron  and  0.008  to  0.16  gm.  of  beryl- 
lium oxide;  it  was  acidified  with  2  drops  of  concentrated  sulphuric 
acid.  The  current  strength  was  0.4  to  0.6  ampere  at  the  start  and 
the  potential  6.5  to  6.8  volts;  toward  the  last  the  current  was  1.2 
to  0.8  ampere.  Owing  to  the  increase  in  the  acidity  during  the 
electrolysis,  the  conductivity  of  the  solution  becomes  better  toward 
the  last.  The  electrolysis  required  14  hours. 

Separation  of  Iron,  Beryllium  and  Aluminium.* 

The  iron  is  deposited  upon  the  cathode  under  precisely  the  same 
conditions  as  those  just  given.  After  the  deposition  of  the  iron 
is  complete,  the  solution  is  poured  into  another  platinum  dish 
and  the  electrolysis  continued  until  all  the  oxalate  is  decomposed, 
and  the  aluminium  is  precipitated  as  hydroxide. 

Inasmuch  as  the  precipitate  may  contain  small  quantities  of 
beryllium  hydroxide,  it  is  advisable  to  dissolve  it  in  as  little  oxalic 
acid  as  possible,  to  add  about  3  gms.  of  ammonium  oxalate  and 
to  electrolyze  again  until  the  aluminium  is  reprecipitated  by  the 
current. 

Separation  of  Iron  from  Aluminium,  Uranium,  and  Rare  Earths 
with  a  Mercury  Cathode  and  Rotating  Anode. 

By  the  combination  of  a  mercury  cathode  of  small  dimensions 
and  a  rotating  spiral  anode,  L.  G.  Kollock  and  E.  F.  Smith  f 
effected  this  separation  with  a  great  economy  of  time.  As  elec- 
trolyzing  vessel  a  test  tube,  3.5  cm.  wide  and  7.5  cm.  tall,  was 
used.  The  bottom  was  flattened  out  and  a  platinum  wire  fused 
into  it  (cf.  Fig.  49,  p.  205).  The  anode  was  formed  from  a  stout 
piece  of  platinum  wire  1  mm.  thick,  the  bottom  of  the  wire  was 
wound  into  a  spiral  of  1.5  cm.  diameter  and  the  upper  end  fastened 
to  the  binding  post  of  the  rotating  axis.  The  quantity  of  mercury 
used  was  40  to  50  gms.  and  sufficed  for  two  determinations.  Con- 
cerning the  weighing,  washing  and  drying  of  the  mercury,  see 
page  205.  The  iron  determinations  made  by  the  above  authors 
were  under  the  following  conditions.  As  salts,  the  sulphates  or 
nitrates  were  used  and  the  spiral  anode  made  520  to  900  revolu- 
tions per  minute. 

*  Classen,  Ber.,  14,  2771  (1881). 

t  J.  Am.  Chem.  Soc.,  27,  1255,  1527  (1905). 


274  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

1.  Iron  from  Uranium.    Iron  0.18  gm.;  uranyl  sulphate  0.2  gm.; 
volume  7  cc.;  sulphuric  acid  2  drops  (30  drops  =  1  cc.);  current 
strength  at  the  start  2.5  amperes,  at  the  last  5  amperes;  potential, 
7  volts  at  first,  5  volts  toward  the  last;  time  15  minutes. 

2.  Iron    from    Aluminium.     Aluminium    sulphate    0.2    gm.; 
current  strength  2  and  5  amperes  (see  above);   potential  9  and  7 
volts;  the  other  conditions  as  under  1. 

3.  Iron  from  Thorium.     Thorium  nitrate  0.2  to  0.3  gm.;   cur- 
rent  strength  3  and  4  amperes;   potential  7  and  5  volts;  the 
remaining  conditions  as  under  1. 

4.  Iron  from  Lanthanum.     Iron  0.12  gm.;  lanthanum  sulphate 
0.15  to  0.25  gm.;  volume  10  cc.;  sulphuric  acid  2  drops;  current 
strength  2  and  4  amperes;  potential  8  and  6  volts;  time  15  minutes. 

5.  Iron  from  Praseodymium.     Iron  0.12  gm.;    praseodymium 
sulphate  0.25  to  0.3  gm.;    volume  8  cc.;    current  strength  3  and 
5  amperes;   potential  9  and  6  volts;   time  20  minutes;   otherwise, 
as  under  4. 

6.  Iron  from  Neodymium.     Neodymium  sulphate  0.16  to  0.24 
gm.;  volume  7  to  8  cc.;  current  strength  3  and  5  amperes;  poten- 
tial 9  and  5  volts;  time  20  minutes;  otherwise  as  under  4. 

7.  Iron  from  Cerium.     Cerium  sulphate  0.12  to  0.36  gm.;  vol- 
ume 8  to  10  cc.;   current  strength  2  and  4  amperes;   potential  9 
and  6  volts;   time  20  minutes;  otherwise  as  under  4. 

8.  Iron  from  Zirconium.     Zirconium  sulphate  0.2  to  0.5  gm. ; 
volume  7  to  10  cc.;   current  strength  2  and  5  amperes;  potential 
7  and  5  volts;  time  20  to  25  minutes;  otherwise  as  under  4. 

In  a  similar  manner,  iron  may  be  separated  from  titanium  and 
from  phosphoric  acid. 

Separation  of  Iron  from  Vanadium. 

R.  E.  Myers  *  accomplished  the  electrolytic  deposition  of  iron 
in  the  presence  of  vanadium  by  using  a  mercury  cathode  and  a 
rotating  anode  (cf.  p.  205;,  under  the  following  conditions.  Iron 
0.1  to  0.2  gm.;  vanadium  0.02  to  0.1  gm.;  concentrated  sulphuric 
acid  12  drops  if  considerable  iron  was  present  and  5  drops  if  only 
little  iron  was  present;  current  strength  0.3  to  0.6  ampere; 
potential  7  volts.  Larger  quantities  of  vanadium,  e.g.,  0.2  gm., 
tend  to  prevent  the  complete  deposition  of  the  iron;  this  diffi- 

*  J.  Am.  Chem.  Soc.,  26,  1134  (1904). 


SEPARATION  OF  NICKEL  FROM  ZINC  275 

culty  can  be  overcome  by  diluting  the  solution  and  electrolyzing 
the  diluted  solution  in  separate  portions. 

Separation  of  Iron  from  Lead. 

The  lead  is  deposited  as  peroxide  upon  the  anode  in  the  presence 
of  nitric  acid  (cf.  p.  194),  the  nitric  acid  is  removed  from  the 
solution  by  evaporation  with  sulphuric  acid,  and  the  iron  deter- 
mined in  an  oxalate  solution  according  to  page  183. 

NICKEL. 
Separation  of  Nickel  from  Lead. 

The  electrolysis  is  carried  out  exactly  as  described  for  the 
determination  of  lead  on  page  194. 

Separation  of  Nickel  from  Zinc. 

From  an  ammoniacal  solution  containing  nickel  and  zinc,  the  cur- 
rent, under  ordinary  conditions,  causes  the  deposition  of  an  alloy 
of  these  metals  on  the  cathode.  A.  Hollard  and  L.  Bertiaux  * 
have  found  that  at  90°  and  in  the  presence  of  sulphite  only  the 
nickel  is  deposited  and  the  separation  is  quantitative.  F.  Foer- 
ster  f  explains  why  such  a  separation  is  possible  on  the  basis  of 
measurements  made  by  F.  Blankenberg  on  the  discharge  potentials 
of  nickel  and  of  zinc  in  ammoniacal  solutions  at  different  temper- 
atures. 

In  Fig.  52,  the  discharge  potentials  are  plotted  as  abscissas  and 
the  current  densities,  in  amperes  per  100  sq.  cm.  of  electrode  sur- 
face, are  plotted  as  ordinates. 

The  curve  for  zinc  at  18°  shows  that  in  an  ammoniacal  solution 
the  current  density  can  vary  from  about  0.04  (a)  to  about  0.45  (b) 
while  the  discharge  potential  changes  only  from  1.14  to  1.15  (6). 

The  curve  for  zinc  at  50°  shows  that  if  the  same  solution  is 
heated  to  50°  and  the  current  density  is  changed  from  0.04  (c)  to 
0.45  (d)  the  discharge  potential  will  vary  from  1.09  (c)  to  1.12  (d) 
volts. 

In  general,  it  may  be  said  that  the  discharge  potential  of  zinc 

*  Bull.  soc.  chim.,  31,  102  (1904). 
t  Z.  Elektrochem.,  13,  563  (1907). 


276 


QUANTITATIVE   ANALYSIS   BY  ELECTROLYSIS 


ions  in  ammoniacal  solution  varies  but  little  as  the  solution  is 
heated,  or  as  the  current  density  is  changed. 

The  fact  that  the  same  is  not  true  of  an  ammoniacal  nickel 
solution  is  evident  from  a  study  of  the  other  four  curves.  From 
the  curve  of  nickel  at  18°,  it  is  seen  that  when  the  current  density 
changes  in  a  solution  at  this  temperature  from  0.05  (e)  to  0.45  (/) 
the  discharge  potential  of  the  nickel  rises  from  0.85  (e)  to  0.98  (/) 
volt;  it  is  apparent,  moreover,  that  at  the  lower  current  density, 
NDioo  =  0.05,  there  is  a  difference  between  the  discharge  potentials 
of  zinc  and  of  nickel  amounting  to  a  —  e  =  0.29  volt,  whereas 
the  difference  in  the  discharge  potentials  of  zinc  and  nickel  with 
a  higher  current  density,  NDioo  =  0.45  ampere,  the  difference  is 
only  6  —  /  —  0.17  volt.  This  slight  difference  between  the  dis- 


* 

8 

I 


0.5 


0.4 


0.3 


0.2 


0.5          0.6          0.7          0.8          0.9          1.0  1.1          1.2          1.3    Volt 

Discharge  Potential 

FlG.  52. 

charge  potentials  of  the  two  metals  at  ordinary  temperatures,  and, 
as  we  have  seen,  the  difference  becomes  less  at  higher  current 
densities,  is  the  reason  why  nickel  and  zinc  cannot  be  separated 
from  one  another  by  an  electrolysis  of  an  ammoniacal  solution  at 
the  laboratory  temperature. 

The  relations  are  different,  however,  as  the  temperature  of  the 
bath  is  raised.  The  diagram  shows  that  at  50°  with  current 
density  of  NDioo  =  0.45  the  potential  difference  between  the 
zinc  and  the  nickel  is  d  —  g  =  0.28  and  the  difference  increases 
as  the  temperature  is  raised.  At  90°  the  potential  of  the  nickel 
for  NDioo  =  0.45  ampere  is  only  0.63  volt  (h)  while  that  of  zinc 


SEPARATION  OF  NICKEL  FROM  ZINC  277 

remains  not  far  from  the  value  at  50°;  there  is  then  a  difference 
in  the  discharge  potentials  of  d—  h=  about  0.49  volt,  or  nearly 
three  times  the  difference  at  18°  under  otherwise  similar  con- 
ditions. 

Thus,  because  the  discharge  potential  of  zinc  remains  about 
the  same  at  different  temperatures  while  that  of  nickel  drops 
considerably  as  the  temperature  rises,  the  separation  of  nickel 
from  zinc,  which  is  impossible  at  18°,  can  be  accomplished  at  90° 
(cf.  p.  94). 

If  the  separation  of  nickel  and  zinc  were  attempted  at  90°  with- 
out the  addition  of  any  sulphite,  it  would  take  a  long  time  to 
deposit  the  last  traces  of  nickel.  The  cause  of  the  sulphite  effect 
is  explained  differently  by  Foerster  than  by  Hollard  and  Bertiaux. 
It  is  certain,  however,  that  the  delay  in  the  deposition  of  the 
nickel  is  closely  related  to  reactions  taking  place  at  the  anode, 
and  Hollard  and  Bertiaux  succeeded  in  stopping  this  disturbing 
effect  by  adding  sulphite. 

Foerster  carries  out  the  electrolytic  separation  of  nickel  from 
zinc  as  follows:  The  solution  of  the  sulphates  (such  as  remains, 
for  example,  in  the  analysis  of  German  silver  after  the  electrolytic 
deposition  of  the  copper  according  to  p.  240)  is  neutralized  with 
strong  ammonia.  If  enough  sulphuric  acid  was  already  present 
to  form  about  5  gms.  of  ammonium  sulphate  by  the  neutraliza- 
tion, no  more  is  needed;  otherwise  enough  ammonium  sulphate  is 
added  to  make  about  5  gms.  of  the  salt.  Then  30  to  35  cc.  of 
ammonia  (sp.  gr.  0.91)  and  0.5  to  1  gm.  of  crystallized  sodium 
sulphite  are  added,  the  solution  diluted  to  250  cc.,  and  heated  to 
90°  or  92°.  The  electrolysis  is  carried  out  with  gauze  electrodes 
using  a  current  of  0.1  ampere;  0.15  gm.  nickel  will  be  deposited 
hi  about  2  hours.  When  a  little  of  the  solution  shows  no  brown 
coloration  with  ammonium  sulphide  (or,  better,  no  test  for  nickel 
with  dimethyl  glyoxime)  the  electrodes  are  raised,  while  directing 
a  stream  of  water  against  them  from  the  wash  bottle,  and  the 
weight  of  nickel  is  determined  (p.  186). 

The  determination  of  the  zinc  can  be  accomplished  by  electrolyz- 
ing  the  cold  solution  with  a  current  of  0.3  to  0.5  ampere  using  a 
copper-plated  gauze  cathode.  The  electrode  used  in  the  nickel 
determination  may  be  used  here,  after  the  nickel  deposit  has  been 
given  a  slight  coating  with  copper.  The  deposition  of  0.16  gm. 
of  zinc  requires  about  3  hours. 


278          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

F.  Foerster  and  W.  Treadwell  *  in  testing  the  above  method 
found  that  the  nickel  deposits  contained  sulphur  and  the  error 
increased  with  the  quantity  of  sodium  sulphite  added.  Thus  with 
0.11  gm.  of  nickel  and  2  gms.  of  NaaSOs.?  H20,  the  error  was  from 
2  to  2.6  per  cent  of  the  total  amount  of  nickel.  If  the  nickel 
deposit  contains  as  much  sulphur  as  this,  it  should  be  redissolved 
and  deposited  electrolytically  without  the  addition  of  any  sulphite 
(p.  187). 


Rapid  Separation  of  Nickel  and  Zinc. 

Ths  method  just  described  can  be  used  as  a  basis  for  a  rapid 
separation  provided  an  auxiliary  electrode  (p.  40)  is  used,  as  A. 
Fischer  f  has  proved  by  experiments  in  the  Aachen  laboratory. 

The  solution  containing  about  0.15  of  each  metal  as  sulphate  is 
treated  with  5  gms.  of  ammonium  sulphate,  1  to  3  gms.  of  sodium 
sulphite  and  30  cc.  of  ammonia  (sp.  gr.  0.91),  and  diluted  to  250 
or  300  cc.  The  gauze  electrodes  shown  on  page  66  are  used. 
The  temperature  of  the  electrolyte  is  kept  between  90°  and  92° 
from  start  to  finish,  and  the  current  is  regulated  so  that  the 
potential  difference  between  the  mercurous  sulphate  —  2  N.H2SO4 
electrode  and  the  cathode  is  constant  at  1.35  volts.  To  maintain 
tkis  electromotive  force,  the  current  strength,  which  is  1  ampere  at 
the  start,  must  be  reduced  gradually  until  at  the  last  it  is  only  0.1 
ampere.  (See  pp.  39,  148  for  details  of  the  procedure.)  Under 
these  conditions  the  nickel  is  deposited  in  about  20  minutes. 

In  the  solution  freed  from  nickel,  the  unchanged  sodium  sul- 
phite is  oxidized  by  heating  with  hydrogen  peroxide.  Then  the 
excess  of  ammonia  is  expelled  by  heating  more  strongly,  and  the 
heating  is  continued  with  the  addition  of  sodium  hydroxide  until 
finally  all  the  ammonia  from  the  ammonium  salts  is  expelled. 
In  the  alkaline  solution,  the  zinc  is  determined  according  to  page 
184. 

A.  Fischer  in  testing  the  rapid  method  could  find  only  traces 
of  sulphur  in  the  nickel  deposit  even  under  unfavorable  condi- 
tions. 

*  Z.  Elektrochem.,  14,  89  (1908). 
t  Chem.-Ztg.,  32,  185  (1908). 


RAPID  SEPARATION  OF   NICKEL  FROM   CHROMIUM    279 

Deposition  of  Zinc  in  the  Presence  of  Nickel  from  Alka- 
line-tartrate  Solution. 

According  to  G.  Vortmann  *  the  solution  of  the  two  metals  is 
treated  with  4  to  6  gms.  of  Rochelle  salt,  made  alkaline  with 
concentrated  sodium-hydroxide  solution,  and  electrolyzed  at  the 
laboratory  temperature  with  a  current  density  of  ND10o  =  0.3 
to  0.6  ampere.  The  deposition  of  the  zinc  is  complete  in  from 
2  to  4  hours  according  to  the  quantity  present  (0.02  to  0.1  gm.). 
There  is  no  good  way  of  testing  the  solution  to  see  if  all  the  zinc 
has  been  deposited.  Either  the  cathode  should  be  weighed  and 
the  electrolysis  continued  a  little  longer  to  see  if  any  change  in 
weight  is  experienced,  or,  as  is  not  quite  as  satisfactory,  a  narrow 
strip  of  brass  may  be  suspended  over  the  edge  of  the  cathode  to 
see  if  any  zinc  is  deposited  upon  it. 

To  determine  the  nickel,  the  solution  is  made  slightly  acid 
with  sulphuric  acid,  treated  with  an  excess  of  ammonia,  and 
electrolyzed  according  to  page  185. 

Separation  of  Nickel  from  Chromium. 

The  separation  is  the  same  as  that  of  iron  from  chromium.  It 
is  to  be  remembered  that  the  nickel  deposited  from  oxalate  solu- 
tion is  contaminated  with  carbon  (cf.  p.  190),  and,  for  this  reason, 
should  be  dissolved  in  nitric  acid  and  deposited  in  ammoniacal 
solution  as  described  on  page  185.  The  presence  of  the  carbon  is 
shown  by  the  brown  coloration  of  the  nitric-acid  solution  obtained 
on  dissolving  the  first  deposit. 

Rapid  Separation  of  Nickel  from  Chromium. 

According  to  the  experiments  of  A.  Fischer  in  the  Aachen 
laboratory,  the  solution  of  the  sulphates,  containing  about  0.18 
gm.  of  nickel  and  0.13  gm.  of  chromium,  is  treated  with  10  gms.  of 
ammonium  oxalate,  diluted  to  120  cc.  and  electrolyzed  at  50°,  using 
a  platinum  dish  as  cathode  and  a  disk  making  600  revolutions  per 
minute  as  anode;  the  current  should  be  7.5  amperes  at  5.3  volts. 
During  the  progress  of  the  electrolysis,  the  temperature  rises,  as 
a  result  of  the  heating  effect  of  the  relatively  strong  current,  and 
is  about  85°  at  the  end  of  the  electrolysis,  which  usually  requires 
50  minutes. 

The  nickel  deposit,  on  account  of  its  carbon  content,  is  dissolved 
*  Monatsh.,  14,  546  (1893). 


280          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

in  nitric  acid,  the  solution  changed  to  sulphate  by  evaporating 
with  concentrated  sulphuric  acid,  and,  after  diluting  and  neutral- 
izing with  ammonia,  it  is  treated  with  1.5  gms.  of  ammonium  sul- 
phate and  40  cc.  ammonia  (sp.  gr.  0.91).  The  electrolysis  of  the 
hot  solution  (120  cc.)  is  conducted  with  a  current  of  5  amperes 
and  10  volts  with  the  disk  anode  revolving  about  800  times  per 
minute.  The  deposition  of  the  nickel  requires  about  30  minutes. 
The  chromium  remaining  in  the  solution  after  the  first  electroly- 
sis is  oxidized  completely  to  chromate  by  electrolyzing  the  solu- 
tion at  80°,  with  the  addition  of  5  gms.  more  of  ammonium  oxa- 
late,  and  using  a  current  of  5  amperes  at  7  volts,  while  the  anode 
is  making  about  600  revolutions  per  minute.  The  oxidation  is 
complete  at  the  end  of  50  minutes.  In  case  the  solution  con- 
tains no  nitrate,  the  chromium  can  be  determined  iodometrically. 

Separation  of  Nickel  from  Aluminium  and  Uranium. 

The  method  is  the  same  as  that  employed  in  the  separation  of 
iron  from  aluminium  and  uranium  (p.  269  et  seq.),  but  with  a 
second  deposition  of  the  nickel  (cf.  p.  190). 

COBALT. 

Separation  of  Cobalt  from  Zinc. 

From  4  to  6  gms.  of  Rochelle  salt  are  dissolved  in  the  solution 
of  the  two  metals  and  a  moderate  excess  of  10  to  20  per  cent 
sodium-hydroxide  solution  is  added.  To  prevent  the  deposition 
of  Co203  at  the  anode,  1  or  2  gms.  of  potassium  iodide  are  added: 
the  latter  acts  as  depolarizer  and  lowers  the  anode  potential 
until  the  positive  electricity  at  the  anode  finds  it  easier  to  oxidize 
the  iodine  ion  to  iodate  rather  than  the  cobaltous  ion  to  cobaltic 
oxide.  In  spite  of  this  precaution,  however,  it  is  always  neces- 
sary to  weigh  the  anode  as  some  oxide  usually  deposits  there. 
The  solution  is  heated  to  60°  and  electrolyzed  with  a  current  of 
NDioo  =  0.07  to  0.1  ampere  at  2  volts.  At  the  end  of  the  proc- 
ess, the  cathode  is  weighed  after  the  customary  treatment;  the 
anode  is  rinsed  carefully  with  water,  dried  at  110°  in  an  air-bath, 
and  the  weight  of  cobalt  corresponding  to  the  Co203  found  is 
added  to  the  weight  of  the  metal  on  the  cathode.  This  method 
was  proposed  by  Vortmann*  and  proved  satisfactory  by  the 
experiments  of  A.  Waller  f  in  the  Aachen  laboratory. 

*  Z.  Elektrochem.,  1,  6  (1894-95).  t  Ibid.,  4,  243  (1897). 


SEPARATION  OF  NICKEL  FROM   COBALT  281 

To  determine  the  zinc  the  solution  freed  from  cobalt,  together 
with  the  washings,  is  electrolyzed  with  a  current  of  NDioo  =  0.3 
to  0.6  ampere.  The  electrolysis  is  conducted  in  a  beaker  and  a 
gauze  cathode,  plated  with  copper  or  with  silver,  is  used. 

Separation  of  Cobalt  from  Aluminium,  Chromium  and 
Uranium. 

The  separation  is  accomplished  as  in  the  separation  of  iron  from 
the  last  three  metals  (p.  270).  The  cobalt  deposit  from  the  oxalate 
solution  will  contain  carbon  and  thus  for  accurate  results  it  must 
be  dissolved  and  the  metal  deposited  from  an  ammoniacal  solu- 
tion (cf.  p.  187). 

Separation  of  Nickel  from  Cobalt. 

The  physical  and  chemical  similarity  between  nickel  and  cobalt 
is  so  marked  that  it  is  extremely  difficult  to  devise  a  satisfactory 
electrolytic  separation  and  comparatively  few  attempts  have  been 
made  in  this  direction.  Still,  on  account  of  the  importance  of 
the  matter,  a  few  of  the  researches  will  be  mentioned.  The 
method  recommended  by  D.  Balachowsky  *  is  complicated  and 
the  results  obtained  are  not  accurate  enough  to  satisfy  the  demands 
of  electro-analysis. 

A.  Coehn  and  M.  Glaser  f  deposit  the  cobalt  as  cobaltic  oxide, 
Co2O3.  They  found  that  in  a  slightly  acid  solution  cobalt  is 
deposited  as  oxide  upon  the  anode  while  nickel  is  not;  in  a 
strongly  acid  solution  there  is  no  formation  of  oxide  with  either 
metal.  They  also  found  that  it  was  possible  to  make  the  deposi- 
tion of  the  cobaltic  oxide  quantitative  provided  means  were  taken 
to  prevent  any  deposition  of  metallic  cobalt  upon  the  cathode. 
This  tendency  of  depositing  upon  both  cathode  and  anode  is  a 
property  that  cobalt  shares  with  other  metals  such  as  silver,  lead, 
bismuth  and  manganese. 

There  are  several  ways  to  prevent  the  deposition  of  the  metal 
upon  the  cathode.  An  ion  may  be  provided  in  the  solution  which 
is  more  readily  discharged  than  the  metal  ion  in  question:  thus 
in  the  case  of  cobalt  or  lead,  such  an  ion  is  the  hydrogen  ion. 
The  deposition  of  the  cobalt  may  be  prevented,  therefore,  by 

*  Compt.  rend.,  131,  1492  (1901). 
t  Z.  anorg.  Chem.,  33,  9  (1903) 


282  QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS 

adjusting  the  potential  of  the  electric  current  so  that  hydrogen 
is  barely  liberated  at  the  cathode.  Under  these  conditions  no 
cobalt  is  deposited  because  the  discharge  potential  of  cobaltous 
ions  is  0.22  volt  higher  than  the  discharge  potential  of  hydrogen 
ions.  By  using  a  platinized  cathode  (cf.  p.  82),  the  liberation  of 
hydrogen  is  made  more  easy.  Unfortunately,  however,  under 
these  conditions  (2.3  to  2.4  volts  and  0.01  ampere)  the  formation 
of  the  cobaltic  oxide  takes  place  so  slowly  that  0.08  gm.  of  cobalt 
is  not  precipitated  in  the  course  of  20  hours.  A  further  difficulty 
lies  in  the  fact  that  the  electrolysis  requires  attention:  only  a 
small  deposit  of  oxide  should  be  allowed  to  form  and  for  this 
reason  the  anode  must  be  frequently  changed  during  the  process. 

In  the  electrolysis  ^of  a  lead  solution  it  was  possible  to  increase 
the  concentration  of  hydrogen  ions  by  the  addition  of  a  strong 
acid  and  thereby  facilitate  the  discharge  of  such  ions.  This 
method  is  of  no  avail  in  the  case  of  the  cobalt  determination, 
because  no  cobaltic  oxide  is  precipitated  from  a  strongly  acid 
solution. 

It  will  be  remembered  that  in  the  case  of  the  lead  determination 
some  authors  recommended  adding  copper  ions  to  the  solution,  as 
such  ions  are  discharged  more  readily  than  hydrogen  ions.  In 
this  case,  however,  nickel  is  to  be  determined  in  the  solution  after 
the  cobalt  has  been  deposited  and  thus  the  addition  of  copper  is 
not  permissible. 

A  third  expedient  is  to  remove  the  hydrogen  ions  as  fast  as 
possible  by  adding  a  depolarizer  to  the  electrolyte;  i.e.,  a  sub- 
stance upon  which  the  neutral  hydrogen  will  react  as  fast  as  its 
ions  are  discharged,  and  if  this  reaction  velocity  is  so  great  that 
no  hydrogen  gas  is  evolved,  even  when  high  current  densities 
are  used,  the  desired  end  is  attained. 

Coehn  and  Glaser  have  found  potassium  dichromate  to  be  a 
satisfactory  depolarizer.  Their  method  is  as  follows: 

The  solution,  containing  about  0.08  gm.  of  cobalt  and  0.02  to 
0.08  gm.  of  nickel  is  treated  with  0.1  to  0.2  gm.  of  potassium  dichro- 
mate and  with  3  to  4  gms.  of  potassium  sulphate  to  improve  the 
conductivity  of  the  solution.  After  diluting  to  about  500  cc.,  the 
solution  is  heated  and  electrolyzed  at  a  constant  potential  of  2.3  to 
2.4  volts  (see  p.  212)  and  a  current  strength  of  0.10  to  0.15  ampere 
for  10  hours.  As  cathode  a  platinized  foil  electrode  may  be  used 
(p.  78),  5  X  9.5  cm.  .and  as  anode  an  equally  large  platinum 


SEPARATION  OF  NICKEL  FROM   COBALT  283 

gauze  electrodes.  Two  such  anodes  are  necessary  but  they  are  not 
weighed.  As  soon  as  the  gauze  has  become  covered  with  a  dark 
coating  it  is  taken  out  of  the  solution,  washed  and  replaced  by 
the  other  electrode.  During  the  electrolysis  it  is  necessary  to 
change  the  anode  about  five  times;  at  first,  when  the  solution  is 
more  concentrated,  the  exchange  is  made  at  more  frequent  inter- 
vals than  toward  the  last.  When  no  further  deposit  of  oxide  is 
noticed,  the  electrolysis  is  finished. 

Each  time  the  anode  is  removed  from  the  solution  the  deposited 
oxide  is  dissolved  by  dilute  sulphuric  acid  to  which  a  little  sulphur- 
ous acid  has  been  added.  The  excess  of  the  latter  is  removed 
by  heating  and  the  cobalt  determined  as  on  page  191. 

For  the  nickel  determination  the  chromate  remaining  in  the 
solution  is  reduced  by  heating  with  sulphurous  acid,  the  excess  of 
the  latter  is  removed,  and  the  analysis  is  continued  as  described 
on  page  185. 

Enough  has  been  said  to  show  the  complexity  of  this  method  of 
analysis.  The  cobalt  is  all  precipitated  from  the  solution  but 
the  oxide  usually  contains  a  little  nickel.  To  make  sure  that  no 
nickel  is  present,  the  solution  in  sulphuric  acid,  after  the  removal 
of  the  sulphurous  acid,  should  be  neutralized  with  ammonia,  the 
excess  of  ammonia  removed  by  evaporation  and  the  electrolysis 
carried  out  anew  with  the  addition  of  dichromate,  etc. 

As  compared  with  the  simultaneous  determination  of  nickel 
and  cobalt  from  ammoniacal  solution  and  the  determination  of 
the  nickel  with  dimethyl  glyoxime,  this  method  is  of  little  value 
although  it  does  furnish  a  good  means  of  testing  a  nickel  solution 
to  see  if  any  cobalt  is  present.  The  dilute  solution  of  the  neutral 
salts  is  treated  with  potassium  dichromate  and  potassium  sul- 
phate, heated  to  boiling  and  electrolyzed  between  two  platinum 
wires  for  a  few  minutes  with  a  current  at  2.3  to  2.4  volts.  If  the 
anode  becomes  darkened,  cobalt  is  probably  present  and  can  be 
confirmed  by  the  borax-bead  test. 


284  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

LEAD. 

Separation  of  Lead  from  Other  Metals. 

In  the  electrolytic  determination  of  lead,  the  metal  is  usually 
deposited  as  peroxide  upon  the  anode  (cf.  p.  194) .  The  large  quan- 
tity of  nitric  acid,  which  it  is  advisable  that  the  electrolyte  should 
contain,  makes  it  possible  to  effect  a  separation  from  nearly  all 
other  metals  although  another  metal  cannot  always  be  deposited 
simultaneously  upon  the  cathode.  The  method  that  serves  for 
the  separation  of  small  quantities  of  lead  from  larger  quantities  of 
copper  was  described  on  page  230,  and  the  procedure  to  be  fol- 
lowed when  a  large  quantity  of  lead  is  present  was  also  described. 

According  to  the  researches  of  G.  Vortmann,*  an  accurate 
separation  of  lead  from  other  metals  is  possible  only  in  the  case  of 
copper.  He  found  that  small  quantities  of  other  metals  invariably 
contaminated  the  lead-peroxide  deposit.  These  results  ha^ve  not 
been  confirmed  by  other  investigators  and  further  work  is  needed 
to  settle  the  matter.  It  would  seem  advisable,  however,  for  the 
most  accurate  results,  to  dissolve  the  peroxide  deposit  in  nitric 
acid  to  which  a  little  oxalic  acid  is  added,  and  to  repeat  the  elec- 
trolytic deposition.  In  the  presence  of  arsenic,  tellurium,  silver, 
bismuth  and  chlorides,  the  results  are  too  high,  as  Vortmann  and 
other  experimenters  have  found. 

In  a  solution  containing  15  to  20  per  cent  by  volume  of  free 
nitric  acid  (sp.  gr.  1.35  to  1.38)t  the  lead  can  be  separated  from 
the  following  metals  under  the  experimental  conditions  given  on 
page  194:  alkalies,  aluminium,  barium,  beryllium,  chromium, 
cadmium,  calcium,  cobalt,  iron,  magnesium,  mercury,  nickel, 
strontium,  uranium,  zinc  and  zirconium. 

Separation  of  Lead  from  Antimony. 

There  is  no  known  method  of  separating  these  two  metals 
electrolytically.  According  to  H.  Nissenson  and  B.  Neumann,! 
however,  it  is  easy  to  separate  the  two  elements  chemically  and 
then  determine  each  separately.  For  example,  in.  the  analysis 

*  Ann.  Chem.,  351,  283  (1907). 

t  In  the  presence  of  phosphoric  acid,  this  quantity  of  nitric  acid  does  not 
prevent  the  deposition  of  metallic  lead  upon  the  cathode  (cf.  p.  197). 
J  Chem.-Ztg.,  19,  1142  (1895). 


SEPARATION  OF  LEAD  FROM  ANTIMONY  285 

of  hard  lead,  type  metal,  etc.,  about  2.5  gms.  of  the  borings  are 
placed  in  a  250-cc.  graduated  flask,  10  gms.  of  tartaric  acid,  15  cc. 
of  water  and  4  cc.  of  nitric  acid  (sp.  gr.  1.4)  are  added  and  the  alloy 
is  dissolved  by  heating  the  acid.  The  resulting  clear  solution  is 
treated  with  4  cc.  of  concentrated  sulphuric  acid,  diluted  with  water, 
cooled  and  brought  to  the  graduation  mark.  The  precipitate  of 
lead  sulphate,  owing  to  the  presence  of  the  tartaric  acid,  is  per- 
fectly free  from  antimony,  as  the  authors  have  proved  by  experi- 
ment. The  solution  is  mixed  thoroughly  by  pouring  it  back  and 
forth  several  times  into  a  dry  beaker  and,  after  allowing  the  pre- 
cipitate to  settle,  it  is  filtered  through  a  dry  filter.  An  aliquot 
part  of  the  nitrate  (50  cc.  =  one  fifth)  is  made  strongly  alkaline 
with  caustic-soda  solution,  boiled  with  50  cc.  of  a  saturated  sodium- 
sulphide  solution,  filtered  immediately  and  the  precipitate  washed 
with  water  containing  a  little  sodium  sulphide.  The  precipitate 
usually  consists  of  a  little  copper  sulphide.  The  electrolytic 
deposition  of  the  antimony  is  carried  out  in  a  hot  solution  after 
the  addition  of  potassium  cyanide  (cf.  p.  290). 

If  it  is  desired  to  determine  the  lead  as  well,  the  entire  precipi- 
tate from  2.5  gms.  of  the  alloy  is  too  large  for  the  electrolytic 
method  described  on  page  231.  The  precipitate  is  washed  with 
water,  containing  about  10  gms.  of  tartaric  acid  and  4  cc.  of  con- 
centrated sulphuric  acid  in  250  cc.,  until  free  from  antimony.  The 
tartaric  and  sulphuric  acids  are  removed  by  washing  with  alcohol 
and  the  precipitate  weighed  in  the  usual  manner. 


286  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

MOLYBDENUM. 
Separation  of  Molybdenum  from  Vanadium. 

R.  E.  Myers,*  using  the  mercury  cathode  described  by  E.  F. 
Smith  (p.  205),  obtained  satisfactory  deposits  of  molybdenum 
in  the  presence  of  vanadium  under  the  following  conditions.  In 
one  case  the  solution  contained  0.095  gm.  of  molybdenum  and 
0.1  gm.  of  vanadium  present  as  sodium  molybdate  and  vanadate. 
The  solution  was  acidified  with  20  drops  of  concentrated  sulphuric 
acid  and  electrolyzed  with  a  current  of  1.6  to  2  amperes  at  6.5 
volts  potential.  After  15  or  17  hours  the  free  sulphuric  acid  was 
exactly  neutralized  with  caustic  potash  solution,  15  drops  of  sul- 
phuric acid  added,  and  in  this  way  the  acidity  kept  the  same  at 
the  end  of  the  electrolysis  as  at  the  start.  This  regulation  of  the 
acidity  has  proved  advantageous  for  the  deposition  of  the  last 
traces  of  molybdenum.  The  electrolysis  was  continued  3  hours 
at  1  to  1.5  amperes  at  5.5  volts. 

In  another  case,  0.19  gm.  molybdenum  and  0.01  gm.  vanadium 
were  present  in  the  original  solution.  Then,  the  first  addition  of 
sulphuric  acid  amounted  to  30  drops  and  the  current  was  1.5 
amperes  at  4.5  volts.  After  the  neutralization  with  alkali,  20 
drops  of  acid  were  added  and  the  electrolysis  finished  with  1.2  to 
1.5  amperes  at  5.5  to  6  volts. 

If  the  acidity  of  the  solution  is  cut  down  too  much,  some  oxide 
of  vanadium  will  deposit  on  the  anode.  The  end  of  the  electrolysis 
is  recognized  by  the  green  color  of  the  solution. 

The  molybdenum  deposits  in  the  form  of  a  lustrous,  white 
amalgam  which  is  washed  with  water,  alcohol  and  ether,  dried 
and  weighed  together  with  the  electrolyzing  vessel  (cf.  p.  205). 

*  J.  Am.  Chem.  Soc.,  26,  1130  (1904). 


PART  IV. 

SPECIAL  ANALYSES. 

THE  methods  of  electro-analysis,  up  to  the  present  time,  have 
not  been  adapted  to  the  determination  of  the  composition  of 
substances  of  a  complicated  nature;  they  are  used  more  like  volu- 
metric methods  as  aids  to  ordinary  gravimetric  analysis.  The 
following  examples  of  practical  analyses  should  be  judged  from 
this  viewpoint. 

Analysis  of  Commercial  Copper. 

Method  of  A.  Hollard  and  L.  Bertiaux.  The  following  scheme 
of  analysis  includes  the  determination  of  these  impurities:  lead, 
bismuth,  nickel,  cobalt,  iron,  zinc,  manganese,  tin,  antimony, 
arsenic,  silver  and  sulphur.  Manganese  is  not  a  common  con- 
stituent of  the  metal,  but  is  found  sometimes  as  slag  or  matte 
inclusions. 

In  the  case  of  an  ordinary  analysis,  the  nature  and  quantity  of 
the  impurities  present  are  usually  determined  and  the  percentage 
of  copper  is  determined  by  difference;  if  the  copper  is  determined 
electrolytically,  it  is  only  as  a  check  upon  the  process.  Thus,  an 
error  of,  say,  0.1  per  cent  in  the  determination  of  the  copper  would 
not  be  regarded  as  a  serious  analytical  mistake  in  the  electrolysis, 
but  the  presence  of  this  amount  of  impurity  is,  in  some  cases,  a 
very  serious  matter.  The  electrolytic  deposition  of  copper  is  usu- 
ally carried  out  chiefly  for  the  purpose  of  obtaining  a  solution  in 
which  the  impurities  can  be  determined. 

A  brief  summary  of  the  principal  operations  will  be  given. 

1.  Determination  of  the  Copper.  A  solution  of  the  metal  in 
nitric  and  sulphuric  acid  is  electrolyzed,  with  the  addition  of 
ferric  sulphate  and  lead  sulphate  if  the  original  metal  is  very 
impure.  These  substances  are  added  to  prevent  the  electrolytic 
deposition  of  arsenic,  antimony  and  bismuth  (p.  290).  The 
deposited  copper  contains  all  the  silver  which  is  determined  as 

287 


288          QUANTITATIVE   ANALYSIS    BY   ELECTROLYSIS 

described  below  (8)  and  the  corresponding  weight  deducted  from 
the  weight  of  silver  and  copper. 

The  addition  of  lead  sulphate  only  prevents  the  injurious  effect 
of  small  quantities  of  bismuth.  If  considerable  bismuth  is  present, 
the  lead  and  bismuth  must  be  removed  before  determining  the 
copper,  by  the  precipitation  method  outlined  under  2. 

2.  Determination  of  Lead  and  Bismuth.     The  nitric-acid  solution 
of  another  portion  of  the  metal  is  evaporated  with  sulphuric  acid 
till  it  fumes  strongly;   after  diluting,  the  residue  will  contain  the 
lead  as  sulphate  and  a  part  of  the  bismuth  as  arsenate  and  antimo- 
nate.     The  remainder  of  the  bismuth  is  precipitated  as  phosphate 
by  the  addition  of  phosphoric  acid. 

From  the  nitric-acid  solution  of  the  residue,  the  lead  is  again 
precipitated  by  sulphuric  acid.  The  lead  is  determined  by  elec- 
trolysis and  likewise  the  bismuth  in  the  solution  free  from  let  d. 

If  the  quantity  of  bismuth  present  is  large,  the  copper  is  deter- 
mined in  the  solution  from  which  the  bismuth  was  precipitated 
as  phosphate,  arsenate  and  antimonate. 

3.  Determination  of  Nickel,  Cobalt,  Iron  and  Zinc.    In  a  new 
sample,  the  copper  is  deposited  as  under  1,  but  without  the  addi- 
tion of  any  substances  to  the  acid  bath,  because  in  this  case  it 
makes  no  difference  whether  or  not  a  pure  deposit  of  copper  is 
obtained.     The  four  metals  in  question  are  sure  to  remain  in  solu- 
tion after  the  electrolysis,  and  the  remaining  metals  of  the  copper 
group  are  precipitated  by  the  introduction  of  hydrogen  sulphide 
into  the  slightly  acid  solution.     The  filtered  sulphide  precipitate, 
which  also  contains  the  tin,  is  saved  for  the  determination  of  the 
metals  in  it,  and  the  filtrate  is  used  for  the  separation  of  nickel, 
cobalt,  iron  and  zinc. 

Nickel  and  cobalt  are  determined  together  by  the  electrolysis 
of  an  ammoniacal  solution  in  the  presence  of  precipitated  ferric 
hydroxide. 

The  ferric  hydroxide  is  dissolved  in  oxalic  acid  and  the  iron 
determined  electrolytically. 

The  solution,  in  which  only  the  zinc  remains,  is  transformed  into 
a  citrate  solution  and  the  zinc  deposited  by  the  current. 

4.  Determination  of  the  Tin.     The  sulphide  precipitate  obtained 
under  3   may  contain  some  lead  and  antimony  as  well  as  tin. 
The  sulphides  are  changed  to  oxalates,  as  described  on  page  302, 
and,  after  precipitating  the  antimony  with  hydrogen  sulphide, 


ANALYSIS  OF    COMMERCIAL    COPPER  289 

the  tin  can  be  determined  electrolytically  in  the  oxalic-acid  solu- 
tion. 

5.  Determination  of  Manganese.     The  electrolytic  determination 
of  the  manganese  can  be  effected  in  the  solution  freed  from  zinc 
as  described  under  3. 

6.  Determination  of  Arsenic.     A  sample  of  the  original  metal 
is  dissolved  in  hydrochloric  acid  and  ferric  sulphate,  and  the  arse- 
nious  chloride  distilled  in  a  current   of  hydrogen  chloride   (see 
below). 

7.  Determination  of  Antimony.    The  solution  freed  from  copper, 
as  described  under  1,  may  be  used  for  this  determination.     After 
the  removal  of  the  nitric  acid,  antimony  sulphide  is  precipitated 
by  introducing  hydrogen-sulphide  gas  and  the  antimony  sulphide 
is  extracted  from  the  precipitate  by  treating  it  with  sodium-sulphide 
solution.     This  solution  is  used  for  the  electrolysis. 

8.  Determination  of  Silver.     The  nitric-acid  solution  of  a  por- 
tion of  the  original  metal  is  treated  with  a  little  hydrochloric  acid. 
The  silver  chloride  is  filtered  off,  dissolved  in  potassium  cyanide, 
and  the  resulting  solution  electrolyzed. 

9.  Determination  of  Sulphur.    According  to  G.  L.  Heath  *  the 
sulphur  is  best  determined  gravimetrically  as  barium  sulphate 
in  the  solution  freed  from  copper  electrolytically.     Naturally  no 
ferric  sulphate  should  be  added  to  the  bath  in  this  case. 

Details  of  the  Method  for  the  Analysis  of  Commercial 

Copper. 

1.  Copper.  The  metal  borings  are  freed  from  grease  by 
treatment  with  ether  and  from  particles  of  iron  (from  the  drill) 
by  means  of  a  magnet.  10  gms.  of  the  metal  are  then  dissolved 
in  a  beaker  about  6.5  cm.  in  diameter  and  18  cm.  tall;  this  small 
diameter  of  the  beaker  is  desirable  because  the  glass  is  to  be  used 
as  electrolyzing  vessel  and  considerable  electrode  surface  can  be 
used  without  unnecessarily  diluting,  and  because  the  tall  sides  of 
the  beaker  tend  to  prevent  loss  by  spattering  during  the  solution 
of  the  metal.  A  funnel  is  placed  in  the  beaker  and  the  contact 
between  the  sides  of  the  beaker  and  the  funnel  is  made  air-tight 
with  a  little  water. 

After  covering  the  borings  with  water,  about  0.4  gm.  of  finely 
powdered  lead  sulphate  (see  below),  12  cc.  of  concentrated  sul- 
*  Chem.-Ztg.,  20,  rep.  113  (1896). 


290  QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS 

phuric  acid,  and  25  cc.  of  nitric  acid  (sp.  gr.  1.33)  are  added  and 
the  reaction  is  hastened  by  warming  slightly.  Refined  copper 
usually  dissolves  completely  but  unrefined  metal  usually  leaves 
behind  a  slight  residue  which  can  be  freed  from  copper  by  warm- 
ing for  some  time. 

The  solution  is  diluted  to  about  300  cc.  and  pure  ferric  sulphate, 
corresponding  to  about  0.1  gm.  of  iron,  is  added.  The  ferric 
salt  helps  to  keep  the  arsenic  in  the  quinquevalent  condition  and 
thus  prevents  its  deposition  upon  the  cathode  with  the  copper. 
The  addition  of  lead  sulphate  has  been  found  by  Hollard  and 
Bertiaux  to  prevent  the  deposition  of  antimony  and  bismuth  on 
the  cathode  while  the  copper  is  being  deposited;  the  deposition  of 
the  lead  peroxide  upon  the  anode  favors  the  deposition  of  bismuth 
upon  the  anode  as  peroxide.  If  considerable  bismuth  is  present, 
the  expedient  of  adding  lead  sulphate  is  of  little  avail  and  it  is 
necessary  to  remove  the  bismuth  as  described  under  2^  before 
attempting  to  determine  the  copper. 

After  the  solution  has  become  clear,  the  gauze  electrodes  are 
introduced  and  the  electrolysis  is  conducted  with  a  current  of 
1  ampere;  at  the  end  of  7  or  8  hours  most  of  the  copper  will  have 
been  deposited.  When  the  blue  color  of  the  solution  has  dis- 
appeared, a  little  water  is  added  to  the  bath  and  it  is  noticed 
whether  there  is  any  further  deposit  of  copper  formed  on  the  freshly 
exposed  electrode  surface  (cf.  p.  125).  The  complete  removal  of 
the  copper  requires  not  more  than  12  hours.  Then,  without 
turning  off  the  current,  the  beaker  is  quickly  removed  from  be- 
neath the  electrodes  and  replaced  by  another  beaker  containing 
distilled  water.  This  wash  water  is  added  to  the  main  solution 
which  is  used  for  the  further  analysis  (see  7).  The  further  treat- 
ment of  the  copper  is  as  described  on  page  99,  but  it  must  be 
borne  in  mind  that  all  the  silver  is  present  and  its  weight  must  be 
deducted. 

2.  Lead  and  Bismuth.  The  solution  of  10  gms.  of  copper  in  50  cc. 
nitric  acid  is  evaporated  to  dryness  with  the  addition  of  10  cc.  of  sul- 
phuric acid.  The  residue  is  treated  with  5  cc.  of  sulphuric  acid 
and  200  cc.  of  water;  the  undissolved  part  contains  lead  sulphate, 
bismuth  arsenate  and  bismuth  antimonate.  If  much  bismuth 
is  present,  some  of  it  remains  in  solution,  so  that  10  cc.  of  phos- 
phoric acid  (60°  Be*.  =  sp.  gr.  1.90)  are  added  to  precipitate  the 
remaining  bismuth  as  phosphate.  The  precipitate,  containing  all 


ANALYSIS   OF   COMMERCIAL   COPPER  291 

the  lead  and  bismuth,  is  allowed  to  settle,  is  filtered  after  standing 
12  hours,  and  washed  with  dilute  sulphuric  acid.  (The  filtrate 
may  be  used  for  the  determination  of  copper  as  in  1.) 

The  precipitate  containing  the  lead  and  bismuth  is  dissolved  by 
heating  with  nitric  acid  and  a  little  hydrochloric  acid,  7  cc.  of  con- 
centrated sulphuric  acid  are  added,  and  the  solution  is  evaporated 
until  fumes  are  evolved.  The  residue,  after  cooling,  is  treated 
with  100  cc.  of  water,  a  little  alcohol  is  added,  and  the  precipitate, 
which  now  consists  wholly  of  lead  sulphate,  is  filtered  off.  After 
washing  with  water  containing  sulphuric  acid  and  alcohol,  the 
lead  sulphate  is  dissolved  and  electrolyzed  according  to  page  197. 

The  solution,  from  which  the  lead  sulphate  was  precipitated 
last,  is  diluted  to  300  cc.  and  the  bismuth  deposited  upon  the 
cathode  by  electrolyzing  with  a  current  of  0.1  ampere;  the 
alcohol  present  does  no  harm.  This  method  is  suitable  only  for 
the  determination  of  small  quantities  of  bismuth  and  requires 
considerable  time  (about  24  hours).  If  the  necessary  apparatus 
is  available,  the  method  described  on  page  147  et  seq.  is  to  be  pre- 
ferred. 

3.  Nickel,  Cobalt,  Iron,  Zinc.  Without  the  addition  of  any 
lead  sulphate  or  ferric  sulphate,  5  gms.  of  the  copper  are  dissolved 
as  under  1,  and  the  greater  part  of  the  copper  is  deposited  electro- 
lytically  while  the  four  metals  in  question  remain  in  solution. 
If  a  deposit  of  lead  peroxide  forms  upon  the  anode,  it  may  con- 
tain some  iron.  It  is  dissolved,  therefore,  in  the  solution  from 
which  the  copper  has  been  removed,  adding,  if  necessary,  a  lit- 
tle hydrogen  peroxide.  The  solution  is  evaporated  to  remove 
the  nitric  acid  and  finally  diluted  with  water.  By  introducing 
hydrogen  sulphide  into  the  hot,  slightly  acid  solution,  all  the 
members  of  the  copper  and  arsenic  groups  are  precipitated  as 
sulphides.  The  precipitate  is  filtered,  washed  and  set  aside  for 
the  determination  of  tin  as  in  4. 

The  filtrate  is  boiled  to  remove  hydrogen  sulphide  and  the  last 
traces  of  this  gas  are  oxidized  by  adding  hydrogen  peroxide,  and 
at  the  same  time  the  ferrous  ions  are  converted  into  ferric  ions. 
The  separation  of  the  nickel  and  cobalt  from  the  zinc  is  accom- 
plished in  an  ammoniacal  solution  in  which  ferric  hydroxide  is 
suspended  (p.  284).  In  preparing  the  electrolyte,  the  excess  of 
hydrogen  peroxide  must  be  removed  by  boiling  before  the  sulphite 
is  added. 


292  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS 

The  nickel  may  be  determined,  if  necessary,  by  the  dimethyl- 
glyoxime  method  and  the  cobalt  determined  by  difference.  For- 
merly it  was  customary  to  determine  the  cobalt  by  the  potassium- 
cobaltic  nitrite  method  and  to  get  the  nickel  by  difference. 

The  ferric  hydroxide  is  filtered  off,  freed  from  any  zinc  by  re- 
peatedly dissolving  in  hydrochloric  acid  and  precipitating  by 
ammonia,  and  the  iron  is  determined  by  titration  or  by  the  elec- 
trolytic method  described  on  page  183. 

The  ammoniacal  liquid  is  used  for  the  determination  of  the  zinc 
as  described  below. 

If,  as  is  the  case  with  certain  very  impure  grades  of  copper,  more 
than  2  per  cent  of  iron  is  present,  some  ferric  hydroxide  may  con- 
taminate the  deposit  of  nickel  and  cobalt.  In  such  a  case  the 
deposit  is  dissolved  in  nitric  acid,  the  solution  evaporated  with 
sulphuric  acid,  and  in  this  way  a  solution  is  obtained  containing  so 
little  iron  that  there  is  no  danger  of  contamination  when  the 
nickel  and  cobalt  are  deposited  again  from  an  ammoniacal  solution. 

All  the  solutions  from  which  the  nickel  and  cobalt  have  been  pre- 
cipitated are  combined,  freed  from  ferric  hydroxide  (purifying  the 
precipitate  as  mentioned  above)  and  concentrated  by  evaporation. 
According  to  Hollard  and  Bertiaux  the  solution  is  prepared  for  the 
zinc  determination  as  follows:  After  neutralizing  the  sulphuric 
acid,  5  gms.  of  sodium  hydroxide  in  excess  are  added,  followed  by 
2  gms.  of  citric  acid,  and  the  excess  of  the  alkali  is  neutralized 
with  sulphuric  acid.  Then  the  solution  is  made  slightly  alkaline 
with  a  few  drops  of  sodium  hydroxide  and  finally  2  gms.  more  of 
citric  acid  are  added.  The  solution  is  diluted  to  400  cc.  and  the 
zinc  deposited  upon  a  coppered  platinum  cathode  with  a  current 
of  1  ampere. 

4.  Tin.     The  sulphide  precipitate  obtained  under  2  is  heated 
with  sodium  sulphide  to  dissolve  the  tin  and  free  it  from  sulphides 
of  copper,  etc.     The  filtered  solution  is  decomposed  by  introducing 
hydrogen-sulphide  gas  and  the  tin  sulphide  is  dissolved  in  oxalic 
acid;   since  antimony  is  also  present  in  this  solution  it  must  be 
precipitated  by  the  introduction  of  more  hydrogen  sulphide  into 
the  hot  solution.     The  presence  of  oxalic  acid  prevents  the  pre- 
cipitation of  the  tin  (see  p.  302). 

5.  Manganese.    The  determination  of  manganese  can  be  com- 
bined with  that  of  zinc,  if  ScholPs  method,  described  on  pages  201 
and  289  is  used. 


COPPER  IN   MATERIALS   RICH   IN   IRON  293 

6.  Arsenic.     1  to  5  gms.  of  copper  are  dissolved  in  the  presence 
of  about  8  times  as  much  pure  ferric  sulphate  and  heated  with 
150  cc.  of  concentrated  hydrochloric  acid  in  a  distilling  flask.     The 
escaping  vapors  are  led  into  water  and  the  distillation  is  continued, 
until  the  contents  of  the  flask  have  become  nearly  dry.     In  this 
way  all  the  arsenic  trichloride  is  distilled  over  into  the  water. 
The  arsenic  trichloride  solution  thus  obtained  is  neutralized  with 
ammonia,  made  slightly  acid  with  hydrochloric  acid  and  titrated 
with  iodine  in  the  presence  of  sodium  bicarbonate;  or,  the  arsenic 
may  be  precipitated  as  sulphide  and  determined  gravimetrically. 
Care  must  be  taken  to  run  a  blank  on  the  acid  used,  as  most 
commercial  hydrochloric  acid  contains  arsenic. 

7.  Antimony.     The  solution  freed  from  copper  under  1  is  used 
for  the  antimony  determination  after  any  deposited  lead  peroxide 
has  been  dissolved  off  the  anode.     The  nitric  acid  is  removed  by 
evaporation,   the  solution  is  diluted  and  hydrogen  sulphide  is 
introduced  in  the  cold;    under  these  conditions  the  arsenic  will 
remain  in  solution.     The  sulphide  precipitate  on  being  treated 
with  sodium  sulphide  yields  a  solution  from  which  the  antimony 
may  be  deposited  electrolytically  by  the  method  given  on  page 
158. 

8.  Silver  and  9.   Sulphur,  see  page  289. 

Determination  of  Copper  in  Materials  Rich  in  Iron. 

The  difficulties  involved  in  the  determination  of  copper  in  a 
solution  containing  considerable  iron  (cf.  pp.  127,  237)  were 
found  particularly  annoying  by  Fairlie  and  Bone  (cf.  p.  72)  in  the 
rapid  electrodeposition  of  copper.  Thus,  if  a  solution  obtained 
from  3  gms.  of  copper  slag,  containing  40  to  50  per  cent  ferrous 
oxide,  is  electrolyzed  at  a  volume  of  about  150  cc.  in  the  presence 
of  2  cc.  or  more  of  nitric  acid  with  a  current  of  3  amperes  and 
rotating  anode,  the  copper  at  once  begins  to  deposit  upon  the 
cathode.  At  the  same  time  the  solution  begins  to  assume  a  brown 
color  and  the  color  deepens  until  nearly  all  the  copper  is  deposited. 
Then  the  solution  suddenly  becomes  colorless  and  the  copper 
begins  to  dissolve  in  the  electrolyte  which  now  contains  only  a 
very  little  ferrous  iron. 

The  cause  of  the  brown  color  lies  in  the  fact  that  some  of  the 
ferric  iron  is  reduced  to  ferrous  iron  and  unites  with  nitric  oxide 
which  is  present  to  some  extent  as  a  reduction  product  of  the  nitric 


294  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS 

acid.  This  compound,  of  variable  composition,  between  ferrous 
salt  and  nitric  oxide,  is  formed  in  the  well-known  qualitative  test 
for  nitric  acid.  As  long  as  the  ferrous  salt  is  forming,  the  deposi- 
tion of  the  copper  Continues,  because  ferrous  ions  exert  no  solvent 
effect  upon  the  copper  deposit.  As  soon,  however,  as  the  unstable 
compound  of  ferrous  salt  with  nitric  oxide  decomposes,  which 
change  is  recognized  by  the  solution  suddenly  becoming  colorless, 
all  the  iron  is  oxidized  back  into  ferric  salt  and  this  exerts  a  solvent 
action  upon  the  deposit.  This  solvent  action  is  considerably  in- 
creased, as  a  number  of  investigators  have  found,  if  considerable 
nitric  acid  is  present  at  the  same  time.  If  little  nitric  acid  is 
present,  the  solvent  effect  of  the  ferric  salt  is  scarcely  noticeable. 
Thus,  if  only  1  cc.  of  nitric  acid  is  present  in  the  electrolyte  an 
accurate  determination  of  the  copper  can  be  accomplished. 

To  determine  the  copper  in  a  copper  slag  rich  in  iron,  3  gms.  of 
the  finely  powdered  substance  are  mixed  with  15  cc.  of  hydro- 
chloric acid  (sp.  gr.  1.2)  in  a  porcelain  dish,  and  5  cc.  of  nitric  acid 
(sp.  gr.  1.42)  are  added.  After  heating  for  about  5  minutes,  the 
solution  is  allowed  to  cool  somewhat,  4  cc.  of  concentrated  sul- 
phuric acid  are  added,  and  the  solution  is  evaporated  until  white 
fumes  of  sulphuric  acid  are  evolved.  After  cooling,  30  cc.  of  water 
and  exactly  1  cc.  of  concentrated  nitric  acid  are  added,  and 
the  solution  is  boiled  and  filtered.  The  filtrate  is  diluted  to  150  cc., 
heated  to  50°  or  55°  and  electrolyzed  with  a  current  of  NDioo  = 
3  amperes  with  the  anode  making  400  to  500  revolutions  per 
minute  (see  p.  128).  The  deposition  of  the  copper  requires  about 
35  or  40  minutes. 

The  method  to  be  followed  when  other  disturbing  elements, 
such  as  arsenic  and  antimony,  are  present,  is  described  on 
page  287. 

If  the  solution  contains  only  a  little  iron,  the  presence  of  more 
nitric  acid  does  no  harm,  as  in  the  analysis  of  the  following 
materials  (cf.  p.  237). 

Crude  Converter  Copper.  5  gms.  of  copper  are  dissolved  in 
21  cc.  of  nitric  acid  (sp.  gr.  1.42),  5  cc.  of  concentrated  sulphuric 
acid  are  added,  and  the  solution  is  evaporated  to  dryness.  After 
the  addition  of  2.5  cc.  of  nitric  acid,  the  residue  is  dissolved  in 
water,  diluted  to  150  cc.,  and  electrolyzed  at  50°  to  55°  with  a  cur- 
rent of  NDioo  =  5  amperes.  The  anode  makes  500  revolutions 
per  minute  and  the  electrolysis  requires  about  2  hours. 


COPPER   IN  MATERIALS   RICH   IN   IRON  295 

Copper  Matte.  One  gram  of  the  powdered  material  is  dissolved 
in  15  cc.  of  strong  nitric  acid  containing  bromine  and  allowed  to 
stand  without  heating  until  all  the  sulphur  is  oxidized.  Then  4  cc. 
of  concentrated  hydrochloric  acid-  are  added,  the  decomposition  is 
completed  by  gently  boiling,  and,  after  the  addition  of  2  cc.  of 
concentrated  sulphuric  acid,  the  solution  is  evaporated  to  dryness. 
The  dry  residue  is  moistened  with  2  cc.  of  sulphuric  acid  and  2  cc. 
of  nitric  acid,  diluted  with  water  to  150  cc.,  and  electrolyzed  at  50° 
to  55°  with  a  current  of  NDioo  =  3  amperes  in  45  to  55  minutes. 
The  anode  revolves  as  in  the  preceding  case  (cf.  p.  128). 

Ores.  I  gm.  of  the  powder  is  heated  with  10  cc.  of  nitric  acid 
and  the  solution  is  evaporated  to  dryness  with  1  cc.  of  sulphuric 
acid.  To  the  residue,  3  cc.  of  sulphuric  acid  and  2  cc.  of  nitric 
acid  in  water  are  added,  and  the  filtered  solution  is  diluted  to  150 
cc.  The  electrolysis  is  carried  out  at  50°  to  55°  with  a  current  of 
NDioo  =  2  amperes;  the  anode  makes  400  to  500  revolutions  per 
minute;  time  required,  40  to  50  minutes. 

The  filtration  of  the  solution  can  be  omitted  in  most  cases  of 
technical  analysis.  The  presence  of  suspensions  does  no  harm 
when  the  electrolysis  is  carried  out  with  a  rotating  electrode  (cf. 
p.  238). 

If  it  is  desired  to  obtain  a  copper  solution  free  from  iron  in  the 
analysis  of  ores  low  in  copper,  instead  of  hydrogen  sulphide  or 
sodium  thiosulphate  to  precipitate  the  copper,  the  use  of  metallic 
aluminium,  as  recommended  by  Heidenreich,*  gives  satisfaction. 

From  2  to  5  gms.  of  ore  are  dissolved  in  a  deep  porcelain  dish 
with  40  to  100  cc.  of  a  mixture  of  3  volumes  of  concentrated  nitric 
acid  and  1  volume  of  concentrated  hydrochloric  acid;  the  action 
is  assisted  by  gentle  heating  until  all  the  sulphide  present  is  oxi- 
dized. The  nitric  acid  is  all  removed  by  twice  evaporating  to 
dryness  with  hydrochloric  acid.  The  residue  is  moistened  with 
5  cc.  of  dilute  hydrochloric  acid  and  dissolved  in  10  cc.  of  water. 
The  solution  is  filtered  into  a  flask  and  diluted  to  about  100  cc. 
A  strip  of  metallic  aluminium  is  added  and  the  solution  heated 
for  a  few  minutes.  The  deposited  copper  is  filtered  off,  washed 
with  hot  water  till  free  from  chloride,  and  the  filter  together  with 
the  deposited  copper  is  ignited  in  a  porcelain  crucible.  The 
residue  is  dissolved  in  dilute  nitric  acid  and  the  filtered  solution 
electrolyzed  according  to  page  123. 

*  Z.  anal.  Chem.,  40,  15  (1901). 


296  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

H.  Danneel  determines  the  copper  in  ores  very  quickly  by  first 
depositing  this  metal  in  a  nitric-acid  solution  without  paying  any 
attention  to  the  other  metals  present  or  to  the  spongy  nature  of 
the  deposit.  In  this  way  all  the  copper  together  with  some  im- 
purity is  obtained.  The  deposit  is  washed  without  interrupting  the 
current,  taking  care  not  to  lose  any  copper.  It  is  not  necessary 
to  wash  very  carefully,  and  one  washing  will  answer  all  purposes. 
The  spongy  copper  in  the  platinum  dish  is  covered  with  dilute 
nitric  acid  and  the  dish  is  now  made  the  anode  in  another  elec- 
trolysis, using  a  gauze  or  sieve  cathode.  In  the  second  deposi- 
tion, the  copper  is  obtained  sufficiently  pure  for  most  analytical 
purposes.  If  the  ore  contains  arsenic,  it  is  well  to  dissolve  the  first 
deposit  in  nitric  acid  and  deposit  the  copper  the  second  time  from 
an  ammoniacal  solution. 

Brass. 

Weigh  1  gm.  of  the  alloy  into  a  slender  beaker  of  about  125-cc. 
capacity.  Add  8  cc.  of  6-normal  nitric  acid,  cover  the  beaker 
and  heat  gently  till  all  the  brass  is  dissolved. 

If,  owing  to  the  presence  of  a  little  tin,  a  white  precipitate  of 
metastannic  acid  forms,  evaporate  to  dryness  but  do  not  bake 
the  residue.  Add  2  cc.  of  nitric  acid,  25  cc.  of  water  and  boil 
gently  to  dissolve  the  nitrates  of  copper  and  zinc.  Filter  off 
the  metastannic  acid,  wash  it  with  hot  water  and  weigh  as  Sn02 
after  ignition  in  a  porcelain  crucible. 

Dilute  the  clear  solution  of  the  nitrates  to  about  75  cc.  and 
boil  gently  for  one  minute  to  remove  any  nitrous  oxides.  Then 
wash  down  the  sides  of  the  beaker  and  the  cover  glass  and  elec- 
trolyze,  using  preferably  a  platinum  gauze  cathode  and  a  rotating 
platinum  gauze  anode.  Use  a  current  of  about  NDioo  =  3 
amperes.  More  current  can  be  used,  but  there  is  then  danger 
of  slight  mechanical  loss  and  more  likelihood  of  obtaining  a 
spongy,  non-adherent  deposit.  It  is  advisable  to  keep  the  beaker 
covered  with  a  split  watch  glass. 

Usually  a  slight  deposit  of  lead  peroxide  will  form  upon  the 
anode.  If  the  alloy  contains  more  lead  than  is  commonly  met 
with  in  brass,  it  is  better  to  deposit  the  lead  peroxide  upon  a 
stationary  gauze  cathode  and  in  that  case  a  rotating  crucible 
cathode  gives  satisfactory  results. 

After  the  solution  has  become  colorless  continue  the  elec- 


BRASS  297 

trolysis  a  few  minutes  longer,  add  0.1  gm.  of  urea  and  wash  down 
the  sides  of  the  beaker.  This  serves  to  expose  fresh  electrode 
surface  to  the  bath,  as  well  as  to  wash  down  any  copper  solution 
that  may  have  spattered  up  from  the  solution.  With  a  little 
practice  one  can  usually  estimate  closely  the  time  when  all  the 
copper  is  deposited.  It  is  not  desirable  to  continue  the  elec- 
trolysis any  longer  than  necessary.  The  bath  gradually  becomes 
neutral,  owing  to  the  reduction  of  nitric  acid,  and  then  zinc 
will  deposit.  Moreover,  enough  nitrous  acid  may  be  formed 
to  dissolve  some  of  the  deposited  copper.  To  counteract  this 
last  influence  the  urea'Ss  added:  if  nitrous  acid  is  present  the  solu- 
tion effervesces  and  nitrogen  gas  is  evolved. 

When  all  the  copper  is  deposited,  it  is  important  to  wash  both 
electrodes  while  breaking  the  circuit.  The  best  way  to  accom- 
plish this  is  to  siphon  off  the  solution  while  pouring  fresh  water 
into  the  beaker,  continuing  until  the  resistance  of  the  bath  be- 
comes so  great  that  an  incandescent  light  in  the  circuit  glows 
very  faintly.  This  treatment,  however,  greatly  dilutes  the 
zinc  solution  and  retards  any  further  work  with  it.  It  is  suf- 
ficient to  wash  the  electrodes  in  this  manner:  Slowly  lower  the 
beaker  with  the  left  hand  while  washing  both  electrodes  with 
a  stream  of  water  from  the  wash  bottle  held  in  the  right  hand. 
Then  quickly  replace  the  original  beaker  with  one  containing 
pure  water.  Turn  off  the  current,  remove  the  electrodes  and 
treat  them  in  the  usual  manner. 

To  make  sure  that  all  the  copper  has  been  deposited,  add 
a  slight  excess  of  ammonia  to  the  entire  solution.  There  should 
be  no  evidence  of  blue  color.  If  one  is  obtained,  discharge  the 
color  by  the  careful  addition  of  sulphuric  acid  and  electrolyze 
again  with  clean  electrodes. 

In  the  commercial  analysis  of  brass,  the  zinc  is  usually  deter- 
mined by  difference.  It  may  be  determined  electrolytically  by 
the  method  of  L.  H.  Ingham  or,  more  suitably,  gravimetrically 
by  precipitation  as  zinc  ammonium  phosphate  in  neutral  solu- 
tion (cf.  Treadwell-Hall,  Analytical  Chemistry,  Vol.  II.) , 

Copper  Matte  (Lead  Matte). 

These  metallurgical  products  contain  silicic  acid,  sulphur,  ar- 
senic, antimony,  iron  (nickel,  cobalt,  zinc),  as  well  as  copper  and 


298  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

lead  which  are  often  the  only  metals  to  be  determined.  1  gm.  of 
the  substance  is  dissolved  in  about  30  cc.  of  nitric  acid  (sp.  gr.  1.4). 
If  it  is  desired  to  determine  the  lead,  the  nitric-acid  solution  is 
diluted  with  hot  water,  filtered  and  electrolyzed  with  the  platinum 
dish  as  anode  (cf.  p.  194). 

If  only  the  copper  is  to  be  determined,  the  solution  should  con- 
tain but  little  nitric  acid  on  account  of  the  disturbing  effect  of  the 
large  quantity  of  iron  present  (cf.  p.  293).  The  solution,  irrespec- 
tive of  whether  the  metal  was  dissolved  in  nitric  acid  containing 
bromine  (cf.  p.  295)  or  in  aqua  regia,  is  evaporated  with  sulphuric 
acid  and  the  sulphate  residue  is  dissolved  in  the  small  volume  of 
nitric  acid  recommended  on  page  295.  In  this  solution  the 
copper  is  determined  slowly  (p.  123)  or  rapidly  (p.  128). 

If  both  lead  and  copper  are  to  be  determined,  the  former  metal 
is  deposited  as  peroxide  as  described  above.  Meanwhile  a  part 
of  the  copper  deposits  upon  the  cathode.  This  copper  deposit 
is  redissolved  in  the  solution  which  has  been  freed  of  lead,  the 
large  excess  of  nitric  acid  is  neutralized  with  ammonia,  and  the 
copper  is  deposited  according  to  page  123.  The  large  quantity  of 
ferric  salt  present  prevents  any  interference  of  the  arsenic  (cf. 
p.  290).  If  the  matte  contained  antimony,  some  lead  sulphate 
must  be  added,  for  the  reason  given  on  page  290.  Any  silver 
present  is  determined  by  dissolving  the  copper  deposit  in  nitric 
acid,  precipitating  with  hydrochloric  acid,  and  weighing  the  silver 
chloride. 

Bronzes. 

A  pure  bronze  contains  only  copper  and  tin  but  the  qualitative 
examination  of  commercial  bronzes  shows  such  differences  in 
purity  that  it  is  impossible  to  give  suitable  procedures  to  cover 
all  cases.  There  are  bronzes  which  are  very  pure  (telephone  and 
telegraph  wire),  being  prepared  from  electrolytic  copper  and 
Banca  tin  and  containing  a  little  phosphorus,  silicon,  or  similar 
constituent,  to  increase  the  tenacity.  The  ordinary  alloys,  on  the 
other  hand,  may  contain  all  the  impurities  present  in  the  copper, 
tin  or  old  metal  which  may  have  been  used  in  their  manufacture. 
Then  there  are  special  bronzes,  such  as  manganese  bronze,  phos- 
phor bronze,  etc.,  to  which  one  or  more  foreign  elements  have 
been  added  designedly.  Since  such  Alloys  are,  as  a  rule,  decom- 
posed by  the  action  of  nitric  acid,  the  important  point  that  first 
arises  is  the  purity  of  the  metastannic  acid  that  is  left  behind  as 


BRONZES  299 

insoluble  residue.  Metastannic  acid  has  the  property  of  carrying 
down  with  it  such  elements  as  copper,  lead,  iron,  arsenic,  antimony 
and  phosphorus,  and  when  the  highest  accuracy  is  demanded  this 
is  a  matter  that  causes  considerable  complication  in  the  analysis. 
The  first  thing  to  be  decided,  therefore,  is  whether  it  is  desired  to 
determine  the  tin  content  with  the  greatest  possible  accuracy 
or  whether  it  will  suffice  to  regard  the  washed  metastannic  acid 
as  pure.  The  contamination  of  the  metastannic  acid  by  other 
oxides  can,  in  the  analysis  of  bronzes,  easily  amount  to  one  per 
cent  or  more;  but  in  many  cases,  when  phosphorus  and  antimony 
are  absent,  it  is  satisfactory  in  technical  analysis  to  leave  the 
impurities  out  of  consideration.  When  antimony,  arsenic  or  phos- 
phorus is  present  it  is  quite  another  matter.  All  but  a  trace  of 
the  former  will  be  precipitated  with  the  tin  and  all  the  phosphorus 
and  arsenic  will  be  precipitated,  provided  six  or  eight  times  as 
much  tin  is  present  in  the  alloy  as  the  weight  of  the  phosphorus 
pentoxide  or  arsenic  pentoxide  formed  by  the  action  of  the  nitric 
acid  on  the  alloy.  Since  one  part  by  weight  of  phosphorus  gives 
more  than  two  parts  by  weight  of  phosphorus  pentoxide,  and  as 
much  as  one  per  cent  of  phosphorus  is  often  present,  it  is  obvious 
that  considerable  error  may  be  introduced  if  the  phosphorus  in 
the  insoluble  residue  is  not  determined. 

Leaving,  for  the  present,  the  phosphorus  bronzes  out  of  con- 
sideration and  remembering  that  it  is  not  common  to  find  much 
arsenic  or  antimony  in  a  bronze,  it  may  be  said  that  in  most 
cases  of  commercial  analysis  it  is  satisfactory  to  wash  the  meta- 
stannic acid  with  water  containing  nitric  acid,  ignite  it  and  weigh 
it  as  stannic  oxide,  SnO2.  In  this  way  results  are  obtained  more 
quickly  than  by  determining  the  tin  electrolytically. 

A  number  of  methods  have  been  proposed  for  obtaining  a  pure 
residue  of  metastannic  acid,  or  for  purifying  it.  To  free  it  from 
iron,  Kiinzel  boils  the  metastannic  acid  with  dilute  sulphuric 
acid.  It  must  be  borne  in  mind,  here,  that  if  large  quantities  of 
iron  are  present  some  tin  may  dissolve  in  the  nitric  acid  (Rose). 
Busse's  method,*  which  was  devised  for  the  analysis  of  tin  alloys 
containing  about  4  per  "cent  of  tin,  consists  in  treating  the  alloy 
with  very  concentrated  nitric  acid  (sp.  gjr.  1.5),  and  the  reaction 
is  started  by  gradually  adding  a  little  water;  the  method  does  not 
give  good  results  if  much  tin  or  impurity  is  present.  The  direct 
*  Z.  anal.  Chem.,  17,  63  (1878). 


300  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

purification  of  the  metastannic  acid  by  means  of  the  electric  cur- 
rent is  discussed  on  page  244. 

To  determine  the  tin  content  of  an  impure  metastannic  acid, 
the  filter  together  with  the  washed  metastannic  acid  is  placed  in  a 
flask,  a  small  piece  of  pure  sodium  hydroxide  is  added,  this  is 
covered  with  a  little  water,  and  then,  on  gently  heating,  the  stannic 
oxide  will  dissolve  to  form  sodium  stannate,  provided  too  large 
an  excess  of  sodium  hydroxide  is  not  used.  By  saturating  this 
solution  with  hydrogen  sulphide,  sodium  thiostannate  is  formed 
and  sulphides  of  copper,  lead,  and  iron  are  left  behind.  The 
black  sulphide  residue  is  filtered,  washed,  and  the  filtrate  is  acidified 
with  acetic  acid  whereby  stannic  sulphide  is  precipitated  together 
with  arsenic  and  antimony  sulphides,  when  these  elements  are  pres- 
ent in  the  original  alloy.  If  to  this  mixture  of  suspended  sulphides 
there  is  added,  without  filtering,  a  hot  solution  of  equal  parts 
oxalic  acid  and  ammonium  oxalate  (3.5  gms.  oxalic  acid  and 
3.5  gms.  oxalate  for  each  0.1  gm.  of  tin  present)  then  the  tin  sul- 
phide will  dissolve,  forming  a  brown  solution  from  which,  after 
filtering,  the  tin  can  be  determined  electrolytically  (cf.  p.  160); 
a  slight  turbidity  of  sulphur  does  no  harm  (F.  Henz).* 

The  precipitated  copper,  lead,  and  iron  sulphides  obtained  in 
purifying  the  metastannic  acid  are  ignited  together  with  the 
filter  in  a  porcelain  crucible;  the  residue  is  dissolved  in  dilute 
nitric  acid  and  added  to  the  solution  obtained  by  treating  the 
original  alloy  with  nitric  acid.  The  copper  is  then  determined 
according  to  page  123.  The  other  metals  present  are  determined 
by  methods  already  given,  and  the  phosphorus  by  precipitation 
(cf.  Treadwell-Hall,  Quantitative  Analysis). 

Alloys  of  Lead,  Tin,  Antimony  and  Copper. 

According  to  the  methods  to  be  described,  the  following  alloys 
may  be  analyzed. 

1.  Alloys  containing  less  than  20  per  cent  tin,  such  as  impure 
tin  foil. 

2.  Solder  composed  of  lead  and  tin,  containing  more  than  20 
per  cent  tin,  with  possibly  some  antimony. 

3.  Britannia  metal  and  similar  alloys  of  tin,  antimony  and 
copper. 

4.  Bronzes  of  copper  and  tin;  phosphor  bronzes. 

*  Z.  anorg.  Chem.,  37,  40  (1903). 


ALLOYS  OF  LEAD,   TIN,   ANTIMONY  AND  COPPER     301 

5.  Bearing  metal  (Babbitt  metal)  of  tin,  lead,  antimony  and 
copper. 

6.  Type  metal  of  lead,  tin  and  antimony. 

7.  Antimonial  lead   (hard  lead)   of   lead   and   antimony  with 
traces  of  copper,  arsenic,  nickel,  cobalt  and  iron.* 

A  solution  is  prepared  of  500  cc.  water,  25  gms.  potassium 
chloride,  400  cc.  concentrated  hydrochloric  acid,  and  100  cc.  nitric 
acid  (sp.  gr.  1.4).  This  solution  decomposes  only  slightly  in  the 
cold. 

1  gm.  of  alloy  is  heated  with  70  to  100  gms.  of  the  acid  solution. 
When  the  action  of  the  acid  upon  the  metal  is  over,  the  solution 
is  concentrated  to  about  50  cc.  and  cooled.  After  the  greater  part 
of  the  lead  has  deposited  as  chloride,  gradually,  under  constant 
stirring,  100  cc.  of  95  per  cent  alcohol  are  added  and  the  solution  is 
allowed  to  stand  for  20  minutes,  to  allow  all  the  lead  chloride  to 
crystallize.  The  solution  is  decanted  through  a  weighed  Gooch 
crucible,  the  crystals  washed  three  times  by  decantation  with  a 
mixture  of  4  volumes  of  alcohol  and  1  volume  of  concentrated 
hydrochloric  acid,  after  which  the  crystals  are  all  transferred  to 
the  crucible  and  freed  from  hydrochloric  acid  by  washing  with 
absolute  alcohol.  The  crucible  is  dried  at  120°  to  130°,  cooled 
in  a  desiccator  and  weighed. 

If  it  is  desired  to  determine  the  lead  electrolytically,  which  is 
advisable  if  the  percentage  is  low,  the  chloride  is  transformed  to 
nitrate  by  treatment  with  nitric  acid  and  the  electrolysis  con- 
ducted as  described  on  page  194. 

The  separation  of  the  tin  from  antimony  is  best  accomplished 
by  Clarke's  method  which  was  originally  devised  for  the  separa- 
tion of  these  two  elements  only,  but,  as  Thompson  has  shown,  it 
may  be  also  used  for  the  separation  of  tin  from  copper.  The 
method  is  based  upon  the  different  behavior  of  the  metals  toward 
hydrogen  sulphide  in  a  boiling  solution  of  oxalic  acid;  antimony 
and  copper  are  precipitated  as  sulphides  while  the  tin  remains  in 
solution.  This  method  is  particularly  suited  here  because  the 
oxalic-acid  solution  can  at  once  be  electrolyzed  for  tin. 

The  solution  freed  from  lead  is  evaporated  to  dryness  on  the 
water  bath  to  remove  the  alcohol  and  hydrochloric  acid;  the 

*  This  classification  of  alloys  was  suggested  by  C.  W.  Thompson  (J.  Soc. 
Chem.  Ind.,  15,  179  (1896)).  Various  details  mentioned  by  Thompson  are 
stated  in  the  above  text. 


302          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

residual  salts  are  dissolved  in  10  gms.  oxalic  acid,  10  gms.  ammo- 
nium oxalate  and  about  200  cc.  of  water.  The  solution  is  heated 
to  boiling  and,  while  it  is  kept  at  a  temperature  near  the  boiling 
point,  a  current  of  hydrogen  sulphide  is  introduced  for  about 
45  minutes.  The  sulphides  of  antimony  and  copper  are  promptly 
filtered  off  and  washed  with  hot  water  containing  hydrogen  sul- 
phide. 

After  the  excess  of  hydrogen  sulphide  has  been  boiled  off,  the 
filtrate  is  concentrated  as  much  as  necessary  and  the  electrolytic 
determination  of  tin  carried  out  as  described  on  page  160. 

The  precipitate  of  copper  and  antimony  sulphides  is  heated  with 
a  little  sodium-sulphide  solution  and  the  treatment  is  repeated 
with  successive  portions  of  this  reagent  until  all  the  antimony  is 
extracted.  The  solution  is  filtered  and  the  antimony  determined, 
after  destroying  the  polysulphide  present,  as  described  on  page 
158. 

The  copper  sulphide  is  dissolved  in  nitric  acid,  and,  after  filter- 
ing off  any  sulphur  that  is  deposited,  the  copper  is  determined 
electrolytically  as  described  on  page  123. 

Inasmuch  as  a  little  lead  invariably  escapes  precipitation  as 
chloride,  the  anode  is  weighed  in  this  last  electrolysis  and  the  last 
traces  of  lead  will  be  deposited  upon  it  as  peroxide  while  the 
copper  is  being  deposited  on  the  anode. 

White  Metals. 

These  alloys,  comprising  Babbitt  metal  as  well  as  other  bearing 
and  antifriction  metals,  contain  tin  as  the  principal  ingredient  in 
the  presence  of  small  quantities  of  antimony  and  copper;  to  some 
of  the  alloys  lead  is  added  and  there  are  always  impurities  likely 
to  be  present  as  a  result  of  impure  metals  being  used  in  the  manu- 
facture of  the  alloys. 

If  the  metal  contains  no  lead,  about  0.5  gm.  is  dissolved  in  3  or 
4  cc.  of  hot  concentrated  nitric  acid;  after  cooling,  tne  solution 
is  diluted  cautiously  with  water  and  a  slight  excess  of  sodium- 
hydroxide  solution  is  added.  This  alkaline  mixture  is  poured  into 
50  cc.  of  a  sodium-sulphide  -solution  which  is  saturated  with  the 
salt  at  the  laboratory  temperature;  this  precipitates  the  copper 
as  sulphide  together  with  some  stannous  sulphide.  The  latter  is 
brought  into  solution  by  heating  the  solution  and  adding  sodium 
polysulphide  drop  by  drop,  avoiding  an  excess.  The  stannous 


ANALYSIS  OF  COMMERCIAL  ZINC  303 

sulphide  is  thus  converted  into  sodium  thiostannate,  soluble  in 
water.  The  insoluble  sulphide  residue  is  filtered  and  washed  with 
water  containing  sodium  sulphide.  The  solution  is  saturated  with 
sodium  monosulphide,  Na2S,  treated  with  the  requisite  amount 
of  potassium  cyanide  and  the  antimony  determined  according  to 
page  158. 

The  tin  is  determined  in  the  oxalate  solution,  prepared  as 
described  on  page  160. 

For  the  determination  of  copper  and  any  other  metals  present, 
the  insoluble  sulphides  remaining  after  the  above  treatment  with 
sodium  sulphide  are  dissolved  in  nitric  acid  and  the  analysis  con- 
tinued by  the  separations  that  have  already  been  given. 

If  the  white  metal  contained  lead,  the  alloy  is  dissolved  in  nitric 
acid  to  which  tartaric  acid  is  added,  and  the  determination  of  lead 
and  antimony  accomplished  according  to  page  284,  and  the  tin 
according  to  page  255. 

Analysis  of  Commercial  Zinc. 

As  regards  the  determination  of  the  impurities  and  the  principal 
metal,  the  statements  on  page  287  apply  here;  the  zinc  is  deter- 
mined by  difference. 

A.  Hollard  and  L.  Bertiaux  *  have  adopted  the  following  pro- 
cedure, which  is  a  combination  of  methods  already  known. 

The  presence  or  absence  of  arsenic  is  determined  by  testing  the 
metal  with  arsenic-free  sulphuric  acid  in  a  Marsh  apparatus. 
If  no  arsenic  is  found,  or  not  enough  to  influence  the  determination 
of  any  other  constituent,  10  gms.  of  zinc  are  dissolved  with  an 
equal  quantity  of  pure  copper  in  87  cc.  of  nitric  acid  (sp.  gr.  1.33) 
after  covering  the  metal  with  water.  The  quantity  of  nitric  acid 
is  such  that,  after  the  solution  of  the  metal  is  complete,  about 
12  cc.  of  free  acid  will  remain.  The  solution  is  electrolyzed  with 
a  current  strength  of  0.3  ampere  (cf.  p.  195).  In  the  presence  of 
the  copper  a  much  denser  deposit  of  lead  peroxide  is  obtained  and 
it  adheres  more  firmly  to  the  anode. 

To  determine  the  iron  a  sufficiently  large  quantity  of  the  zinc 
is  dissolved  in  dilute  sulphuric  acid,  out  of  contact  with  the  air, 
and  the  resulting  ferrous  salt  titrated  with  potassium  perman- 
ganate solution.  This  simple  process  is  permissible,  however, 

*  Analyse  des  Metaux  par  Electrolyse. 


304  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

only  when  the  zinc  contains  no  tin  or  other  substance  that  will 
/eact  with  permanganate  in  dilute  solution.  If  tin  is  present, 
after  the  solution  has  been  accomplished  a  few  grams  of  chemically 
pure  zinc  are  added  to  precipitate  the  tin  upon  it. 

If  the  metal  contains  arsenic,  it  must  be  removed,  as  it  inter- 
feres with  the  determination  of  lead.  About  5  gms.  of  zinc  are 
dissolved  by  boiling  dilute  hydrochloric  acid  whereby  the  arsenic 
is  volatilized  as  trichloride.  If  an  insoluble  residue  remains 
behind,  it  is  dissolved  by  adding  a  few  potassium-chlorate  crystals 
and  the  excess  of  chlorine  is  removed  by  boiling.  Into  this  solu- 
tion hydrogen-sulphide  gas  is  conducted  to  precipitate  the  sul- 
phides of  lead,  cadmium,  copper  and  tin.  The  solution  must  not 
be  too  acid  as  otherwise  the  cadmium  is  not  precipitated  as  sul- 
phide; it  is  necessary,  therefore,  to  neutralize  a  part  of  the  acid 
with  ammonia  before  introducing  the  hydrogen  sulphide.  It 
must  be  remembered  that  the  solubility  products  of  zinc  sulphide 
and  of  cadmium  sulphide  are  not  far  apart.  It  is  better  In  this 
xcase  to  have  the  solution  so  slightly  acid  that  some  zinc  sulphide 
precipitates,  rather  than  so  acid  that  some  of  the  cadmium  fails 
to  come  down.  The  filtered  and  washed  sulphide  precipitate 
is  dissolved  by  heating  with  nitric  acid  and,  to  aid  in  the  oxida- 
tion of  the  sulphur,  some  bromine  water  is  added.  The  result- 
ing solution  is  evaporated  to  dry  ness  on  the  water  bath,  the 
residue  moistened  with  nitric  acid,  diluted  with  water,  and 
the  deposited  metastannic  acid  filtered  off.  After  dissolving  in 
oxalic  acid  and  ammonium  oxalate,  the  tin  is  determined  elec- 
trolytically. 

From  the  nitric-acid  solution,  which  may  contain  lead,  cadmium 
and  copper,  the  lead  is  precipitated  as  sulphate  after  evaporating 
with  sulphuric  acid.  After  filtering  and  washing  the  precipitate  in 
the  usual  manner,  it  is  dissolved  in  a  mixture  of  40  cc.  of  ammonia 
(sp.  gr.  0.925),  67  cc.  of  nitric  acid  (sp.  gr.  1.33)  and  enough 
cupric  nitrate  to  correspond  to  10  gms.  of  copper.  This  mixture 
contains  ammonium  nitrate,  in  which  lead  sulphate  dissolves, 
and  it  also  contains  enough  free  nitric  acid  to  permit  the  elec- 
trolytic determination  of  lead  as  peroxide  as  well  as  sufficient 
copper  to  obtain  a  good  deposit  (cf.  p.  195). 

In  the  sulphuric-acid  solution  from  which  the  lead  was  pre- 
cipitated, the  copper  is  determined  by  the  method  given  on 
page  116, 


DETERMINATION   OF   ZINC   IN   ZINC   DUST  305 

Cadmium  may  be  present  in  the  solution  from  which  the  zinc 
was  deposited.  The  solution  is  evaporated  to  expel  the  nitric 
acid  and  the  cadmium  determined  in  cyanide  solution  according 
to  page  176. 

The  iron  is  found  together  with  the  zinc  in  the  solution  from 
which  the  sulphides  of  lead,  cadmium,  copper  and  tin  have  been 
removed.  The  hydrogen  sulphide  is  expelled  by  boiling  the  solu- 
tion and  finally  adding  a  little  concentrated  nitric  acid  whereby 
the  iron  is  also  oxidized  to  the  ferric  condition.  An  excess  of 
ammonia  is  then  added  and  the  precipitated  ferric  hydroxide  is 
dissolved  in  oxalic  acid.  After  the  addition  of  ammonium  oxa- 
late,  the  iron  is  determined  according  to  page  183. 

Determination  of  Zinc  in  Zinc  Dust,  Blue  Powder, 
Flue  Dust  and  Zinc  Ores. 

K.  Jene  *  determines  the  zinc  in  such  products  as  the  above  by 
the  sodium-zincate  method  as  recommended  by  v.  Foregger.f 

The  solution  of  about  0.5  gm.  of  the  substance  in  aqua  regia 
is  evaporated  to  dryness  with  1  or  2  cc.  of  sulphuric  acid  (1:1) 
and  the  residue  heated.  In  this  way  the  metals  are  converted 
to  sulphates  in  which  state  it  is  necessary  to  have  the  zinc.  The 
sulphates  are  dissolved  by  boiling  water,  the  insoluble  residue 
removed  by  filtration,  and  the  solution  treated  with  4  to  7  gms. 
of  sodium  hydroxide.  This  solution  is  electrolyzed  at  50°  in  a 
copper-coated  platinum  dish  by  means  of  a  current  of  NDi00  = 
1  ampere  at  3.8  to  4.2  volts,  no  attention  being  paid  to  the  pre- 
cipitated hydroxides  of  iron,  manganese,  etc.  (cf.  p.  307).  After 
1J  hours  a  bright  piece  of  copper  foil  is  suspended  over  the  edge 
of  the  dish  and  it  is  noticed  whether  a  deposit  of  zinc  is  formed 
upon  it.  If  after  15  minutes  this  is  not  the  case,  the  deposited 
zinc  is  washed  without  interrupting  the  current,  rinsed,  dried 
and  weighed. 

Owing  to  the  evaporation  of  water  during  the  electrolysis  a 
black  rim  may  form  where  the  zinc  comes  in  contact  with  the 
air.  To  prevent  this,  water  is  added  to  the  dish  from  time  to 
time.  The  zinc  deposit  dissolves  readily  in  dilute  nitric  acid 
before  the  layer  of  copper  is  attacked  so  that  it  is  possible  to  use 
the  dish  for  another  zinc  determination  without  depositing  more 
copper  upon  it. 

*  Chem.-Ztg.,  29,  803  (1905).  f  Inaug.-Dissert.,  Bern,  1896. 


306  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Sphalerite. 

The  method  tested  by  Ingham  (p.  168)  for  depositing  zinc  in 
an  ammoniacal  solution  with  the  aid  of  a  rotating  anode  was  used 
by  him  for  the  determination  of  zinc  in  a  sample  of  sphalerite. 
Of  the  powdered  ore,  0.5  gm.  is  moistened  with  water  in  a  porcelain 
dish  and  heated  slightly  on  the  sand  bath  with  the  addition  of 
nitric  acid  (sp.  gr.  1.42).  When,  at  the  end  of  about  20  minutes, 
the  action  of  the  acid  has  ceased,  the  watch  glass  covering  the  dish 
is  rinsed  with  water  and  raised  somewhat  by  placing  a  glass 
triangle  under  it.  The  contents  of  the  dish  are  then  evaporated 
to  dryness,  the  residue  moistened  with  hydrochloric  acid  (sp.  gr. 
1.21),  again  evaporated  to  dryness,  and  the  operation  repeated 
several  times  to  remove  all  the  nitric  acid.  Finally  the  residual 
salt  is  moistened  with  strong  hydrochloric  acid  and  dissolved  in 
hot  water.  After  the  iron  has  been  precipitated  with  ammonia, 
the  precipitate  containing  ferric  hydroxide  and  gangue  is  firtered, 
dissolved,  without  washing,  in  hydrochloric  acid  and  the  iron  again 
precipitated  with  ammonia.  For  this  entire  operation  about  10  cc. 
of  hydrochloric  acid  should  suffice;  the  quantity  of  ammonia 
added  should  be  regulated  so  that  at  the  last  about  2  cc.  remain  in 
excess. 

The  combined  nitrates  are  treated  with  0.5  gm.  of  ammonium 
chloride  and  the  solution  electrolyzed  at  125  cc.,  while  still  warm, 
with  a  current  of  5  amperes  at  6  volts  (cf.  p.  168).  The  zinc 
forms  as  a  light  gray  deposit  and  adheres  well  enough  to  prevent 
loss  on  washing.  The  results  are  very  satisfactory. 

As  has  already  been  mentioned  several  times,  it  cannot  be 
assumed  as  a  general  rule  that  the  presence  of  suspensions  in  an 
electrolyte  has  no  effect  upon  the  purity  of  a  metal  deposit  nor 
that  any  of  the  metal  present  as  inclusion  in  the  precipitate  (i.e., 
zinc  in  ferric  hydroxide)  will  be  dissolved  out  during  the  progress 
of  the  electrolysis.  As  regards  the  effect  of  the  precipitate  upon 
the  purity  of  a  deposit,  it  is  not  serious  if  the  precipitate  is  not 
allowed  to  remain  in  permanent  contact  with  the  electrode;  thus 
the  electrolysis  in  a  stirred  electrolyte  gives  better  results  when 
suspensions  are  present  than  when  a  stationary  electrolyte  is 
used.  As  regards  the  second  point,  there  are  so  many  contradic- 
tions in  the  literature  that  it  is  advisable  to  exercise  precaution 
in  all  cases. 


LEAD  307 

According  to  Hollard  and  Bertiaux,  the  copper  present  in  meta- 
stannic  acid  is  removed  during  the  progress  of  the  electrolysis; 
these  authors  also  found  that  nickel  in  an  ammoniacal  solution 
could  be  determined  accurately  in  the  presence  of  suspended  ferric 
or  aluminium  hydroxide.  Similarly  K.  Jene  (p.  305)  deposits 
zinc  from  an  alkaline  solution  of  sodium  zincate  containing  hydrox- 
ides of  iron  and  manganese  in  suspension  and  obtains  accurate 
results.  On  the  other  hand,  Ingham  has  found  it  impossible  to 
obtain  accurate  results  in  the  zinc  determination  when  the  elec- 
trolysis of  the  ammoniacal  solution  was  carried  out  in  the  presence 
of  suspended  ferric  hydroxide.  He  found  that  the  precipitate 
invariably  contained  zinc  which  could  not  be  obtained  by  the 
action  of  the  electric  current.  It  is  worth  noting,  however,  that 
Ingham  only  carried  out  the  electrolysis  for  20  minutes  whereas 
Jene  allowed  the  current  to  act  about  seven  tunes  as  long  (cf. 
p.  238). 

Lead  (Refined  or  Soft  Lead). 

The  impurities  likely  to  be  present  include  antimony,  tin, 
arsenic,  silver,  bismuth,  copper,  cadmium,  zinc,  iron,  nickel  and 
cobalt.  Since  the  sum  of  all  these  impurities  seldom  amounts 
to  more  than  a  fraction  of  one  per  cent,  it  is  necessary  to  take  a 
large  sample  for  analysis. 

In  a  two-liter  calibrated  flask,  200  gms.  of  the  finely  cut  metal 
are  treated  with  1275  cc.  of  water,  and  325  cc.  of  nitric  acid 
(sp.  gr.  1.4);  the  dilute  acid  is  heated  until  all  the  metal  is  dis- 
solved. To  the  cooled  solution,  62  cc.  of  concentrated  sulphuric 
acid  are  added,  the  solution  is  cooled  again,  diluted  up  to  the  mark 
and  the  precipitated  lead  sulphate  allowed  to  settle.  The  solution 
is  filtered  and  1750  cc.  of  the  filtrate  are  evaporated  to  dryness  in 
a  porcelain  dish.  The  dry  residue  is  extracted  with  a  little  water 
and  the  insoluble  residue  transferred  to  a  filter  and  washed  with 
water;  in  this  way  a  residue  (A)  and  a  filtrate  (B)  are  obtained. 

The  residue  (A)  consists  chiefly  of  lead  sulphate.  To  extract 
small  quantities  of  antimony  from  it,  it  is  rinsed  into  a  small 
flask,  and  treated  with  sodium  hydroxide  and  25  cc.  of  saturated 
sodium-sulphide  solution.  After  heating  a  short  time,  the  solution 
containing  the  antimony  as  sodium  thioantimonate  (F)  is  filtered 
and  set  aside  for  the  time  being. 

The  filtrate  (B),  which  contains  all  the  other  impurities,  is  acid- 


308          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

ified  with  a  little  hydrochloric  acid,  and  treated  with  hydrogen- 
sulphide  gas  until  the  liquid  above  the  precipitated  sulphides  has 
become  perfectly  clear.  Filtration  now  gives  a  precipitate  (C) 
and  a  filtrate  (D). 

The  precipitate  (C)  contains  the  metals  antimony,  tin,  arsenic, 
silver,  bismuth,  copper  and  cadmium.  It  is  heated  with  sodium- 
sulphide  solution  and  filtered.  This  filtrate  is  combined  with  the 
above-mentioned  filtrate  (F)  leaving  behind  a  residue  (E). 

Antimony,  tin,  arsenic.  If  no  arsenic  is  present  the  solution  is 
ready  for  the  electrolytic  separation  of  tin  and  antimony  after 
treatment  with  potassium  cyanide  and  sodium  hydroxide  (see 
p.  252). 

If,  however,  arsenic  is  present  it  is  best  to  determine  this  element 
in  a  separate  sample  (see  below)  and  to  prepare  a  solution  of  anti- 
mony and  tin,  free  from  arsenic,  in  the  following  manner.  The 
three  sulphides  are  precipitated  by  adding  dilute  sulphuric  acid 
to  the  solution  of  the  soluble  thio  salts.  After  filtering,  the  pre- 
cipitate is  washed  with  water  and  the  arsenic  pentasulphide 
extracted  by  treatment  with  ammonium-carbonate  solution  at 
the  ordinary  temperature.  The  sulphides  of  antimony  and  tin 
are  then  redissolved  in  sodium-monosulphide  solution  and,  after 
treatment  with  potassium  cyanide  and  sodium  hydroxide,  the 
solution  is  electrolyzed  as  described  on  page  252. 

The  residue  (E),  obtained  from  the  second  treatment  with 
sodium-sulphide  solution,  and  which  contains  the  sulphides  of 
silver,  bismuth,  copper  and  cadmium,  is  boiled  with  aqua  regia. 
After  diluting,  any  residue  of  silver  chloride  is  filtered  off. 

Inasmuch  as  a  little  lead  is  likely  to  be  present  in  this  solution, 
it  is  next  evaporated  to  dryness  with  the  addition  of  a  few  drops 
of  sulphuric  acid.  The  residue  is  extracted  with  water  and  the 
lead  sulphate  is  removed  by  filtration. 

'Bismuth.  In  the  filtrate,  containing  bismuth,  copper  and  cad- 
mium, the  bismuth  is  obtained,  after  neutralization  with  ammonia, 
by  adding  ammonium  carbonate.  If  it  is  desired  to  determine 
the  bismuth  electrolytically,  the  bismuth  carbonate  precipitate  is 
dissolved  hi  nitric  or  sulphuric  acid  and  the  electrolysis  conducted 
according  to  page  145,  or  page  149.  A  satisfactory  gravimetric 
method  for  determining  the  bismuth  consists  in  dissolving  the  pre- 
cipitate in  nitric  acid  and  evaporating  to  dryness  in  a  weighed  porce- 
lain crucible.  After  ignition,  bismuth  oxide,  Bi203,  is  obtained. 


LEAD  309 

Cadmium,  copper.  The  ammoniacal  filtrate  from  the  bismuth 
precipitate  is  heated  to  expel  the  ammonia,  and  then  treated  with 
potassium  cyanide.  The  cadmium  is  deposited  electrolytically 
as  described  on  page  176,  and,  after  transforming  the  solvent  to 
nitric  acid,  the  copper  is  determined  according  to  page  123. 

Zinc.  The  nitrate  (D),  obtained  from  the  precipitation  with 
hydrogen  sulphide,  may  contain  zinc,  iron,  nickel  and  cobalt. 
The  excess  of  hydrogen  sulphide  is  expelled  by  boiling,  a  little 
bromine  water  is  added  and  then  an  excess  of  sodium  hydroxide. 
The  zinc  stays  in  solution  as  sodium  zincate  and  the  iron,  nickel 
and  cobalt  are  precipitated  as  hydroxides.  The  zinc  may  be 
determined  in  the  alkaline  solution  according  to  the  method  de- 
scribed on  page  166. 

Nickel,  cobalt.  The  washed  hydroxides  of  iron,  nickel  and 
cobalt  arfe  dissolved  in  sulphuric  acid  and  the  nickel  and  cobalt 
determined  in  ammoniacal  solution  according  to  page  187. 

Iron.  After  the  electrolysis  of  the  nickel  and  cobalt,  the  pre- 
cipitate of  ferric  hydroxide  is  filtered  off,  dissolved  in  sulphuric 
acid  and  the  iron  determined  volumetrically  with  potassium 
permanganate  after  reduction  with  zinc. 

Silver.  The  most  accurate  method  for  determining  the  silver 
is  by  cupellation,  as  small  quantities  of  silver  chloride  are  held 
in  solution  by  lead  nitrate.  A  method  for  the  electrolytic  deter- 
mination of  the  silver  in  lead  is  given  on  page  246. 

Arsenic.  To  determine  the  arsenic,  10  gms.  of  lead  are  heated 
with  30  cc.  of  concentrated  sulphuric  acid  until  all  the  lead  is 
converted  into  sulphate.  To  the  cold  liquid,  30  gms.  of  ferrous 
sulphate  are  added  together  with  2  gms.  of  ferric  sulphate  which 
reacts  with  any  sulphurous  acid  that  may  have  been  formed  by  the 
reduction  of  sulphuric  acid.  After  the  addition  of  300  cc.  of  con- 
centrated hydrochloric  acid,  the  arsenic  trichloride  is  distilled  into 
a  receiver  containing  cold  water.  The  arsenic  in  the  distillate  is 
determined  iodometrically  or  as  sulphide  (cf.  p.  293). 

In  computing  all  the  above  results,  except  in  the  case  of  arsenic, 
it  is  to  be  remembered  that  the  solution  obtained  from  200  gms. 
of  metal  was  diluted  to  a  volume  of  2  liters.  The  precipitate  of 
lead  sulphate  occupies  a  volume  of  46  cc.  so  that  the  true  volume 
of  liquid  was  only  1954  cc.  instead  of  2000.  For  the  analysis 
1750  cc.  were  taken,  corresponding  to  JflHf  '  200  =  179.1  gms.  of 
the  original  metal. 


310  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

Hard  Lead. 

The  determination  of  antimony  in  hard  lead  was  outlined  on 
page  218.  If  it  is  desired  to  determine  the  copper,  lead,  etc., 
present  in  the  sulphides  obtained  on  treatment  with  sodium- 
sulphide  solution,  the  precipitate  is  dissolved  in  nitric  acid  and 
the  various  metals  determined  as  in  the  analysis  of  soft  lead. 

Crude  Lead. 

The  impurities  are  the  same  as  those  in  refined  lead  but  they 
are  present  in  larger  quantities  (1  to  4  per  cent).  From  10  to 
50  gms.  of  the  metal  are  weighed  into  a  graduated  liter-flask,  and 
for  each  10  gms.  of  the  metal  60  cc.  of  water  and  16  cc.  of  nitric 
acid  (sp.  gr.  1.42)  added;  10  gms.  of  tartaric  acid  suffice  even  for 
50  gms.  of  the  metal.  After  solution  is  effected,  3  cc.  of  concen- 
trated sulphuric  acid  are  added  for  each '10  gms.  of  lead,  and  the 
lead  sulphate  is  removed  as  described  on  page  307.  An  aliquot 
part  of  the  nitrate  from  the  lead  sulphate  is  concentrated  by 
evaporation;  then  the  determinations  of  antimony,  tin  and  arsenic 
are  carried  out  after  separating  from  other  metals  by  treatment 
with  sodium-sulphide  and  sodium-hydroxide  solutions,  as  in  the 
analysis  of  hard  lead. 

The  sulphides  insoluble  in  sodium-sulphide  solution  are  dis- 
solved in  aqua  regia.  After  diluting  and  filtering  off  the  silver 
chloride,  the  metals  of  the  copper  group  are  precipitated  in  dilute 
hydrochloric-acid  solution  by  the  introduction  of  hydrogen-sul- 
phide gas,  and  thereby  separated  from  the  metals  of  the  iron 
group.  The  analysis  is  continued  as  in  the  case  of  refined  lead 
(p.  307). 

In  computing  the  percentage  of  impurities  present,  it  may  be 
assumed  that  the  volume  of  lead  sulphate  from  10  gms.  of  the 
original  metal  is  2.15  cc.  This  value  is  lower  than  that  given  on 
page  309,  but  in  this  case  the  impurities  are  present  to  a  greater 
extent. 

Silver  is  determined  by  cupellation  of  the  original  metal. 

Iron  Ores,  Iron  and  Steel. 

In  the  analysis  of  these  materials  only  copper  and  nickel  have 
been  determined  electrolytically  to  any  extent.  The  difficulties 
involved  in  the  electrolysis  of  copper  solutions  containing  con- 
siderable iron  have  been  pointed  out  already  (p.  293).  For  deter- 


NICKEL  311 

mining  the  very  low  copper  content  of  an  iron  or  steel,  the  difficulty 
can  be  overcome  in  the  following  manner. 

100  gms.  of  the  filings  or  borings  are  covered  with  200  cc.  of 
sulphuric  acid  (sp.  gr.  1.26);  when  the  action  slows  down,  200  cc. 
more  of  the  same  acid  are  added  and  solution  promoted  by  heating. 
Finally,  after  diluting  to  about  500  cc.,  the  black  residue,  which 
contains  all  the  copper,  is  filtered  off.  The  filter,  with  its  contents, 
is  incinerated  in  a  100-cc.  porcelain  crucible,  the  residue  treated 
with  a  little  strong  hydrochloric  acid,  whereby  all  the  copper  goes 
into  solution,  and  the  contents  of  the  crucible  evaporated  to  dry- 
ness  with  the  addition  of  a  little  concentrated  sulphuric  acid.  The 
residue  is  treated  with  water,  nitric  acid  is  added,  and  the  copper 
is  deposited  in  the  filtered  solution  according  to  page  123. 

If  the  material  analyzed  contains  much  graphite  (gray  cast  iron) 
the  graphitic  residue  may  include  considerable  iron.  In  that  case 
the  contents  of  the  first  filter  are  rinsed  back  into  the  original 
beaker  and  again  treated  with  200  cc.  of  the  sulphuric  acid  for 
half  an  hour  at  the  boiling  temperature.  After  diluting,  the  liquid 
is  filtered  through  the  same  'filter  and  the  process  continued  as 
described.*  The  nickel  determination  is  outlined  on  page  313. 

Nickel. 

In  the  analysis  of  commercial  nickel,  the  metals  nickel,  copper 
and  iron  may  be  determined  electrolytically. 

Five  grams  of  the  metal,  or  more  if  necessary,  are  dissolved  in 
nitric  acid  and  the  excess  of  acid  expelled  by  heating.  The  residue 
is  diluted  with  water,  and  enough  ammonia  is  added  to  dissolve  the 
nickel.  The  precipitated  hydroxides  of  iron  and  aluminium,  which 
contain  some  silicic  acid  f  as  well  as  some  manganese  and  nickel, 
are  filtered  after  heating  a  short  time.  The  precipitate  is  dis- 
solved in  sulphuric  acid  and  the  precipitation  with  an  excess  of 
ammonia  is  repeated;  in  this  way  all  the  nickel  is  obtained  in  the 
ammoniacal  solution.  The  precipitate  (A]  is  set  aside. 

The  combined  ammoniacal  nickel  filtrates  t  are  electrolyzed 

*  H.  Koch,  Z.  anal.  Chem.,  41,  105  (1902). 

t  For  an  accurate  determination  of  the  silicic  acid,  the  residue  obtained  by 
evaporating  off  the  nitric  acid  must  be  heated  till  the  nitrates  are  decomposed 
and  again  dissolved  in  nitric  acid. 

\  Hollard  and  Bertiaux  recommend  the  addition  of  a  few  cubic  centimeters 
of  hydrogen  peroxide  to  the  ammonia  whereby  the  deposition  of  carbon  (from 
carbide)  with  the  nickel  is  said  to  be  prevented. 


312          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

under  the  conditions  given  on  page  186.  The  deposit  (B)  contains 
all  the  nickel,  cobalt  and  copper.  It  is  dissolved  in  nitric  acid 
and  the  copper  determined  according  to  page  123.  In  the  solution 
freed  from  copper,  the  cobalt  is  determined  as  potassium  cobalti- 
nitrite.*  By  deducting  the  weight  of  copper  and  cobalt  from  the 
weight  of  copper,  cobalt  and  nickel  obtained  in  the  first  electroly- 
sis, the  percentage  of  nickel  is  obtained. 

The  precipitate  (A)  which  contains  all  the  iron  together  with 
arsenic  and  antimony  (as  ferric  arsenate  and  antimonate)  is  dis- 
solved in  sulphuric  acid  and  the  solution  united  with  the  electrolyte 
from  which  the  deposit  (B)  was  obtained,  the  latter  being  first 
freed  from  the  excess  of  ammonia  by  boiling  and  made  acid  with 
sulphuric  acid.  This  combined  solution  is  treated  with  hydrogen 
sulphide  to  precipitate  arsenic  and  antimony,  and  in  the  filtrate 
from  these  sulphides  the  excess  of  hydrogen  sulphide  is  expelled. 
The  last  traces  of  the  gas  are  decomposed  by  means  of  bromine 
water,  the  excess  of  the  latter  is  boiled  off,  and  the  solution  is  trans- 
formed into  a  proper  electrolyte  for  iron  by  the  addition  of  am- 
monium oxalate.  The  iron  is  then  determined  electrolytically 
under  the  conditions  given  on  page  183. 

Manganese,  aluminium  and  the  remaining  impurities  are  deter- 
mined in  the  usual  analytical  way  from  the  solution  after  the 
removal  of  the  iron. 

Determination  of  Nickel  in  Nickel  Steel. 

Various  methods  have  been  proposed  for  determining  the  nickel 
electrolytically  in  the  presence  of  precipitated  ferric  hydroxide,  f 
When  much  iron  is  present,  however,  it  is  certain  that  some  nickel 
will  be  retained  by  the  ferric-hydroxide  precipitate  and  the  de- 
posited nickel  will  be  contaminated  by  contact  with  the  precipi- 
tate. Although  these  two  errors  tend  to  compensate  one  another, 
still  the  only  safe  way  is  to  remove  at  least  the  greater  part  of  the 
iron.  The  best  way  of  accomplishing  this  is  by  the  method  of 
Rothe  which  depends  upon  the  fact  that  undissociated  ferric 
chloride  is  very  much  more  soluble  in  ether  than  it  is  in  water, 
whereas  nickel  chloride  is  insoluble  in  ether.  It  is  important  in 

*  Cf.  Treadwell-Hall,  Quantitative  Analysis. 

tHollard  and  Bertiaux,  Analyse  des  Metaux  par  Electrolyse;  Ducru, 
Classen's  Ausgewahlte  Methoden;  Vortmann,  cf.  page  265. 


DETERMINATION   OF   NICKEL  IN   NICKEL  STEEL       313 

carrying  out  the  separation  that  the  solution  should  be  free  from 
suspended  matter  and  for  this  reason  it  is  necessary  to  remove  any 
silicic  acid  at  the  start.  In  many  cases,  therefore,  the  determina- 
tion of  silicon  will  be  combined  with  this  analysis.  Otherwise,  it  is 
easier  to  volatilize  the  silicic  acid  in  the  form  of  silicon  tetrafluoride, 
in  which  case  a  platinum  dish  is  used  in  dissolving  the  sample. 
According  to  the  nickel  content  of  the  steel,  from  2.5  to  5  gms. 
of  borings  are  dissolved  by  heating  with  40  cc.  of  hydrochloric 
acid  (sp.  gr.  1.12)  and  the  solution  is  evaporated  to  dryness  in  a 
platinum  dish,  adding  a  few  drops  of  hydrofluoric  acid  toward  the 
last.*  The  residue  is  dissolved  in  dilute  hydrochloric  acid,  the 
solution  transferred  to  a  porcelain  evaporating  dish,  heated  to 
boiling  and  cautiously  treated  with  2  to  2.5  cc.  of  concentrated 
nitric  acid,  keeping  the  dish  covered  with  a  watch  glass.  The 
solution  is  evaporated  to  sirupy  consistency,  or  until  ferric 
chloride  begins  to  separate,  to  remove  free  chlorine  or  nitric  acid, 
both  of  which  tend  to  decompose  ether.  The  solution,  concen- 
trated to  about  10  cc.,  is  transferred  to  a  separatory  funnel  and 
shaken  with  ether,  f 

The  solution  from  the  ether  separation  is  nearly,  if  not  quite, 
free  from  iron  when  the  process  is  carried  out  properly;  it  contains 
all  the  nickel  and  nearly  all  of  the  other  constituents  of  the  steel,  t 
The  solution  is  heated  on  the  water  bath  to  expel  the  ether  and 
any  copper  present  is  precipitated  by  treating  the  slightly  acid 
solution  with  hydrogen  sulphide,  or  by  boiling  with  sodium  thio- 
sulphate.  The  copper  sulphide  is  filtered  off,  the  solution  freed 
from  hydrogen  sulphide  by  heating,  and  then  evaporated  nearly 
to  dryness  with  the  addition  of  sulphuric  acid.  The  residue  is 
dissolved  in  water  and  a  few  drops  of  sulphuric  acid,  and  the  man- 
ganese is  precipitated  by  a  double  precipitation  with  ammonia 
and  hydrogen  peroxide. 

In  the  combined  filtrates,  the  nickel  is  deposited  slowly  or  quickly 
under  the  conditions  given  on  page  186  et  seq. 

A  slight  coloration  at  the  anode  is  due  to  traces  of  manganese 

*  The  greater  part  of  the  iron  is  in  the  ferrous  condition  but  during  the 
evaporation  considerable  ferric  chloride  is  formed.  This  may  have  a  harmful 
effect  upon  the  platinum  dish. 

t  For  fuller  details  of  the  ether  separation,  see  Treadwell-Hall,  Quantitative 
Analysis. 

%  Blair  has  found  that  molybdenum  follows  the  iron  in  the  analysis  of  alloy 
steels. 


314  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

which  were  not  precipitated  by  the  ammoniacal  solution  of  hydro- 
gen peroxide. 

Chrome-nickel  Steel. 

The  removal  of  the  iron  is  accomplished  as  described  on  page  313. 
Hydrogen  sulphide  is  introduced  into  the  filtrate  and  any  precip- 
itate of  copper  sulphide  is  filtered  off,  dissolved  in  nitric  acid  and 
the  copper  determined  electrolytically  (p.  123).  The  filtrate, 
which  contains  nickel,  chromium  and  manganese,  is  evaporated 
to  dryness  with  sulphuric  acid  and  the  residue  prepared  for  the 
nickel  electrolysis  as  described  on  page  190.  If  it  is  desired  to 
determine  the  nickel  quickly,  the  residue  is  treated  with  a  solution 
of  10  gms.  ammonium  oxalate,  heated  to  80°  and  electrolyzed 
with  a  current  of  6.5  to  7  amperes  at  5.3  volts.  Manganese  is 
then  precipitated  as  dioxide  but  does  not  adhere  to  the  rotating 

anode. 

« 

To  obtain  pure  nickel  from  this  deposit  which  is  usually  con- 
taminated with  manganese,  carbon  and  iron,  the  deposit  is  dis- 
solved in  nitric  acid,  the  excess  of  acid  removed  by  evaporation 
and  the  iron  and  manganese  precipitated  by  treatment  with  am- 
monia and  hydrogen  peroxide.  The  precipitated  ferric  hydroxide 
and  manganese  dioxide  are  dissolved  in  sulphuric  acid  and  a  little 
hydrogen  peroxide,  and  the  precipitation  is  repeated  in  order  to 
remove  traces  of  nickel.  In  the  combined  ammoniacal  filtrate 
the  nickel  is  determined  according  to  page  186. 

For  the  chromium  determination,  the  solution  from  the  first 
nickel  electrolysis,  in  which  the  chromium  is  present  as  chromic 
salt,  is  transferred  to  a  platinum  dish,  to  be  used  as  anode,  and  the 
chromium  is  oxidized  to  chromate  by  electrolyzing  at  60°  with  a 
current  of  5  amperes  at  6  to  9  volts  potential.  The  oxidation 
requires  nearly  an  hour. 

The  solution  of  chromate  is  acidified  with  acetic  acid,  treated 
with  lead  acetate,  and  the  precipitated  lead  chromate  determined 
in  the  usual  analytical  way. 

Tin. 

Of  the  impurities  present  in  commercial  tin,  the  antimony, 
lead,  copper  and  bismuth  may  be  determined  electrolytically. 
The  arsenic,  iron  and  sulphur  should  be  determined  by  the  usual 
methods. 


TIN  315 

For  the  electro-analysis,  5  gms.  or  more  of  the  sample  are  dis- 
solved in  hydrochloric  acid  with  a  little  nitric  acid.  To  separate 
the  sulphides  of  tin,  antimony  and  arsenic  from  the  sulphides  in- 
soluble in  alkaline  sulphide,  the  solution  is  treated  with  an  excess  of 
ammonia,  and  hydrogen-sulphide  gas  is  introduced  until  all  the 
ammonia  has  been  converted  into  monosulphide.  The  sulphides 
of  lead,  bismuth,  copper  and  iron  are  filtered  off,  dissolved  in  acid, 
and  the  first  three  metals  separated  from  iron  by  a  second  pre- 
cipitation with  hydrogen  sulphide,  this  time  having  the  solution 
slightly  acid  with  hydrochloric  acid. 

To  separate  the  lead  from  the  bismuth  and  copper,  the  three 
sulphides  are  dissolved  in  aqua  regia,  the  solution  made  ammo- 
niacal  and  potassium  cyanide  added.  If  hydrogen  sulphide  is 
conducted  into  this  ammoniacal-cyanide  solution,  the  lead  and  bis- 
muth are  precipitated  as  sulphides  while  the  copper  remains  in 
solution  as  ammonium  cuprocyanide.  The  precipitated  sulphides 
are  dissolved  in  aqua  regia  and  the  resulting  solution  evaporated 
with  sulphuric  acid.  To  the  residue,  dilute  alcohol  is  added, 
and  the  lead  sulphate  is  filtered  off.  The  precipitate  is  washed 
with  water  containing  sulphuric  acid  and  alcohol  and  the  lead  is 
determined  as  described  on  page  232. 

In  the  alcoholic  filtrate  the  bismuth  is  determined  under  the 
conditions  given  on  page  291. 

In  the  filtrate  containing  the  iron,  the  hydrogen  sulphide  is 
expelled  by  boiling,  the  ferrous  salt  is  oxidized  to  ferric  salt  by 
bromine  water,  and  the  iron  precipitated  by  ammonia.  The  iron 
may  be  determined  gravimetrically,  •  volumetrically,  or  electro- 
lytically. 

The  copper  is  determined  best  in  a  separate  portion  of  metal. 
About  5  gms.  are  oxidized  by  treatment  with  nitric  acid  and  the 
residue  of  metastannic  acid  is  washed  with  water  containing  nitric 
acid;  the  solution  will  then  contain  the  greater  part  of  the  copper. 
The  metastannic  acid  is  dissolved  by  adding  solid  potassium 
hydroxide  and  water  (cf.  p.  299),  and  hydrogen  sulphide  is  intro- 
duced until  all  the  potassium  hydroxide  has  been  converted  into 
sulphide.  The  copper-sulphide  precipitate  is  filtered  off,  dissolved 
in  nitric  acid,  and  this  solution  united  with  that  obtained  by  the 
original  treatment  of  the  tin  with  nitric  acid.  The  greater  part  of 
the  nitric  acid  is  neutralized  with  ammonia  and  the  copper  deter- 
mined by  electrolysis  under  the  conditions  given  on  page  123. 


316          QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

There  is  no  danger  of  the  deposited  copper  containing  antimony 
or  arsenic  as  these  elements  have  followed  the  tin  in  the  above 
treatment. 

For  the  determination  of  antimony,  another  portion  of  the  tin 
is  taken  and  the  solution  of  the  thio  salts  is  prepared  as  above 
by  treatment  with  nitric  acid,  potassium  hydroxide  and  hydrogen 
sulphide.  The  insoluble  sulphides  are  filtered  off,  the  polysul- 
phides  reduced  by  the  addition  of  potassium  cyanide  (cf.  p.  157) 
and  the  antimony  deposited  electrolytically.  Since  the  antimony 
is  likely  to  contain  tin  if  the  electrolysis  is  continued  long,  the 
deposit  is  dissolved  in  sodium-polysulphide  solution  (obtained  by 
heating  a  little  sodium-monosulphide  solution  with  some  free 
sulphur),  the  solution  decolorized  by  the  addition  of  potassium 
cyanide,  and  the  electrolysis  repeated. 

The  arsenic  is  volatilized  as  trichloride;  5  gms.  of  borings  are 
dissolved  in  150  cc.  of  hydrochloric  acid  in  the  presence  of  50  gms. 
ferric  sulphate  (cf.  p.  293),  and  the  distillation  carried  but  as 

usual. 

Antimony. 

From  5  to  10  gms.  of  the  metal  are  dissolved  in  aqua  regia,  by 
first  covering  the  borings  with  hydrochloric  acid  and  adding 
nitric  acid  in  small  quantities  until  all  the  metal  is  dissolved.  To 
get  the  antimony  in  the  form  of  soluble  thio  salt,  for  the  purpose 
of  separating  it  from  most  of  the  impurities,  the  best  way  is  to 
make  the  solution  ammoniacal  and  then  saturate  it  with  hydrogen- 
sulphide  gas;  in  this  way  the  ammonium  sulphide  is  formed  in  the 
presence  of  the  substances  upon  which  it  is  to  act.  The  precipi- 
tated sulphides  of  copper,  lead,  bismuth,  iron,  etc.,  are  filtered  off. 

It  may  be  mentioned  here  that  it  is  advisable  to  determine 
the  copper  in  another  sample  of  the  metal.  This  is  partly 
because  copper  sulphide  is  not  absolutely  insoluble  in  ammo- 
nium-sulphide solution  and  partly  because  the  potassium-cyanide 
solution  obtained  in  the  further  course  of  the  analysis  is  not 
altogether  suited  for  the  copper  determination. 

After  the  washed  sulphide  precipitate  has  been  dissolved  in 
hydrochloric  acid  and  bromine,  the  excess  of  the  latter  reagent 
is  removed  by  boiling,  and  the  metals  of  the  copper  group  are 
separated  from  those  of  the  iron  group  by  the  introduction  of 
hydrogen  sulphide  into  the  slightly  acid  solution. 

The  precipitated  sulphides  of  copper,  lead  and  bismuth  may 


ANTIMONY  317 

contain  traces  of  antimony  which,  however,  are  removed  together 
with  the  copper  by  the  following  treatment.  The  sulphides  are 
dissolved  again  in  hydrochloric  acid  and  bromine,  the  excess  of 
the  latter  expelled  by  boiling,  and,  after  making  ammoniacal,  an 
excess  of  potassium  cyanide  is  added.  If  hydrogen  sulphide  is 
introduced  into  this  solution,  the  sulphides  of  lead  and  bismuth 
will  be  precipitated.  The  nitrate  containing  the  copper,  and 
possibly  some  antimony,  is  rejected. 

The  sulphides  of  lead  and  bismuth  are  dissolved  in  aqua  regia 
and  the  lead  is  precipitated  as  sulphate  by  evaporation  with  sul- 
phuric acid;  for  the  further  treatment  see  page  232. 

The  filtrate  containing  the  bismuth  is  electrolyzed  as  on  page 
291. 

If  the  original  metal  contained  cadmium,  this  element  will  be 
found  in  the  bismuth  solution.  In  this  case  the  solution  is  neu- 
tralized with  caustic  soda  and  the  separation  of  bismuth  and 
cadmium  accomplished  as  on  page  260. 

In  the  solution  containing  the  iron,  this  metal  is  determined 
as  described  on  page  183.  If  nickel  is  present,  the  process  given 
on  page  265  is  followed. 

Determination  of  the  Copper.  About  5  gms.  of  the  antimony 
sample  are  dissolved  as  described  on  the  preceding  page  and  the 
excess  of  acid  is  expelled  by  heating,  taking  care  to  leave  enough 
acid  to  prevent  the  precipitation  of  antimony  oxychloride  upon 
dilution  with  water.  The  solution  is  poured  into  one  of  sodium 
sulphide  prepared  with  an  excess  of  sodium  hydroxide  and  in  this 
way  the  copper  is  all  precipitated  as  sulphide  while  all  the  anti- 
mony remains  dissolved  as  sodium  thioantimonate.  The  pre- 
cipitated sulphide  is  filtered,  washed  with  water  and  the  copper 
determined  according  to  page  123. 

Copper-manganese. 

The  copper  and  manganese  content  of  this  alloy  have  been 
determined  by  A.  Fischer  and  Reissmann  in  the  Aachen  labora- 
tory. About  0.5  gm.  of  alloy  is  dissolved  in  nitric  acid  (sp.  gr.  1.2), 
using  an  excess  of  15  to  20  cc.,  and  the  solution  is  electrolyzed 
using  a  dish  as  cathode  and  a  rotating  disk,  making  800  revolutions 
per  minute,  as  anode.  The  current  strength  is  adjusted  to  1  am- 
pere and  the  electrolysis  is  conducted  at  the  laboratory  tempera- 
ture (see  table  on  page  128). 


318  QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS 

The  solution,  free  from  copper,  is  neutralized  with  caustic  soda 
and  treated  with  hydrogen  peroxide.  The  precipitate  of  hydrated 
manganese  dioxide  is  filtered  off,  dissolved  in  acetic  acid  with  the 
addition  of  hydrogen  peroxide  and  the  excess  of  the  latter  removed 
by  the  addition  of  a  little  chromic  acid.  In  this  solution  containing 
acetate  and  chromic  salt,  -the  manganese  is  determined  as  de- 
scribed on  page  191. 

Ordinarily,  only  the  manganese  and  copper  are  determined  in 
this  alloy  and  no  attention  is  paid  to  a  small  iron  content.  If 
much  iron  is  present,  the  acetic-acid  solution  obtained  above  is 
neutralized  with  ammonia,  formic  acid  is  added  and  the  manganese 
and  iron  are  determined  as  described  on  page  268. 

Manganese  Silicide. 

From  the  experiments  conducted  by  A.  Fischer  and  Reissmann, 
it  has  been  found  that  the  manganese  may  be  determined  as 
follows:  About  0.5  gm.  of  the  substance  is  weighed  into  a  platinum 
dish  and  moistened  with  hydrofluoric  acid,  sulphuric  acid  and  a 
little  nitric  acid.  After  the  first  action  is  over  the  mixture  is 
heated  and  the  solution  evaporated  to  dryness. 

From  the  aqueous  extract  of  the  residue,  the  iron  is  precipitated 
as  basic  acetate,  the  precipitate  dissolved  in  as  little  nitric  acid  as 
possible,  and  the  basic  acetate  separation  repeated.  The  com- 
bined filtrates  are  treated  with  acetic  acid  and  chrome  alum  and 
the  manganese  is  deposited  electrolytically  "as  described  on  page 
191. 

If  it  is  desired  to  determine  the  iron,  the  basic  ferric  acetate 
may  be  dissolved  in  oxalic  acid  and,  after  neutralization  with 
ammonia,  the  iron  determined  as  on  page  183. 

Determination  of  Mercury  in  Cinnabar. 

The  transformation  of  mercuric  sulphide  into  chloride,  by  heat- 
ing with  aqua  regia,  and  the  replacement  of  the  nitric  acid  with 
hydrochloric  acid  is  a  tedious  process  and  there  is  danger  of 
volatilizing  some  mercury  chloride.  W.  B.  Rising  and  V.  Lenher  * 
have  found  that  a  more  rapid  method  of  effecting  the  solution  of 
natural  as  well  as  artificial  mercuric  sulphide  is  by  the  use  of 
hydrobromic  acid;  and  that  by  adding  potassium  hydroxide  and 
*  J.  Am.  Chem.  Soc.,  18,  96  (1896). 


DETERMINATION  OF  MERCURY  IN  CINNABAR         319 

potassium  cyanide  to  such  a  solution,  an  electrolytic  determina- 
tion of  the  mercury  can  be  made  at  once. 

The  precipitated  sulphide  is  dissolved  in  12  per  cent  hydro- 
bromic  acid  and  the  natural  sulphide  in  20  per  cent  hydrobromic 
acid,  avoiding  an  excess.  The  precipitated  sulphide  dissolves  in 
the  cold  but  when  the  natural  cinnabar  is  contaminated  with 
silicate,  a  long  digestion  at  the  boiling  temperature  is  required. 

When  all  the  mercury  has  dissolved,  the  solution  is  filtered,  if 
necessary,  and  the  free  hydrobromic  acid  is  neutralized  with 
potassium-hydroxide  solution.  After  adding  an  excess  of  potas- 
sium cyanide,  the  solution  is  electrolyzed  with  a  platinum  dish 
and  disk  anode  and  a  current  density  of  NDioo  =  0.025  ampere 
(cf.  p.  136). 

*  J.  Am.  Chem.  Soc.,  18,  96  (1896). 


INTERNATIONAL  ATOMIC  WEIGHTS,  1918. 


Symbol. 

Atomic 
weight. 

Symbol. 

Atomic 
weight. 

Aluminium      .    . 

Al 

27.1 
120.2 
39.88 
74.96 
137.37 
208.0 
11.0 
79.92 
112.40 
132.81 
40.07 
12.05 
140.25 
35.46 
52.0 
58.97 
93.1 
63.57 
162.5 
167.7 
152.0 
19.0 
157.3 
69.9 
72.5 
9.1 
197.2 
4.0 
163.5 
1.008 
114.8 
126.92 
193.1 
55.84 
82.92 
139.0 
207.20 
6.94 
175.0 
24.32 
54.93 
200.6 
96.0 

Neodymium  .  . 

..Nd 

144.3 
20.2 
58.68 

222.4 
14.01 
190.9 
16.00 
106.7 
31.04 
195.2 
39.10 
140.9 
226.0 
102.9 
85.45 
101.7 
150.4 
44.1 
79.2 
28.3 
107.88 
23.00 
87.63 
32.06 
181.5 
127.5 
159.2 
204.0 
232.4 
168.5 
118.7 
48.1 
184.0 
238.2 
51.0 
130.2 

173.5 

88.7 
65.37 
90.6 

Antimony   

...    .Sb 

Neon  

..Ne 

Argon 

A 

Nickel 

Ni 

Arsenic 

As 

Niton  (radium 
Nitrogen  

emanation) 
Nt 
N 

Barium          .    .  . 

Ba 

Bismuth  

Bi 

Boron  

B 

Osmium  

.  .Os 

Bromine 

Br 

Oxygen 

o 

Cadmium 

Cd 

Palladium 

Pd 

Caesium 

Cs 

Phosphorus.  .  . 
Platinum  

P 
Pt 

Calcium. 

Ca 

Carbon.             .  . 

C 

Potassium.  .  .  . 

K 

Cerium  

..Ce 

Praseodymium 
Radium 

Pr 

Chlorine 

Cl 

Ra 

Chromium 

Cr 

Rhodium. 

Rh 

Cobalt 

Co 

Rubidium 

Rb 

Columbium. 

.    .Cb 

Ruthenium.  .  . 
Samarium  .... 

Ru 
Sa 

Cooper 

Cu 

Dysprosium.  .  .  . 
Erbium    

::£ 

Scandium  .... 

Sc 

Selenium  

Se 

Europium 

Eu 

Silicon 

Si 

Fluorine 

F 

Silver 

Ag 

Gadolinium  
Gallium       

Gd 
Ga 

Sodium  

Na 

Strontium  .... 

Sr 

Germanium  

.  .Ge 

Sulfur  

s 

Glucinum  *  

Gl 

Tantalum.  .  .  . 

Ta 

Gold 

Au 

Tellurium 

Te 

Helium 

He 

Terbium  . 

Tb 

Holmium  

Ho 

Thallium  .... 

Tl 

Hydrogen  

H 

Thorium  

Th 

Indium  

In 

Thulium  

Tm 

Iodine 

I 

Tin 

Sn 

Indium 

Ir 

Titanium. 

Ti 

Iron 

Fe 

Tungsten. 

W 

Krypton    

Kr 

Uranium    . 

u 

Lanthanum  
Lead  

La 
.  .Pb 

Vanadium.  .  .  . 

V 

Xenon 

.    Xe 

Lithium 

Li 

Ytterbium  (Neoytterbium) 
Yb 
Yttrium  -       Yt 

Lutecium. 

Lu 

Magnesium.  .    .  . 

Mg 

Manganese  

Mn 

Zinc  

.  Zn 

Mercury  

Hg 

Zirconium  .... 

Zr 

Molybdenum 

Mo 

*  Also  called  beryllium,  as  in  the  text  of  this  book. 

320 


LOGARITHMS  OF  NUMBERS. 


Natural 
numbers. 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

PROPORTIONAL  PARTS. 

8 

9 

10 

0000 

0043 

0086 

0128 

0170 

0212 

0253 

0294 

0334 

0374 

4 

8 

12 

17 

21 

25 

29 

33 

37 

11 

0414 

0453 

0492 

0531 

0569 

0607 

0645 

0682 

0719 

0755 

4 

8 

11 

15 

19 

23 

26 

30 

34 

12 

0792 

0828 

0864 

0899 

0934 

0969 

1004 

1038 

1072 

1106 

3 

7 

10 

14 

17 

21 

24 

28 

31 

13 

1139 

1173 

1206 

1239 

1271 

1303 

1335 

1367 

1399 

1430 

3 

6 

10 

13 

16 

19 

2326 

29 

14 

1461 

1492 

1523 

1553 

1584 

1614 

1644 

1673 

1703 

1732 

3 

6 

9 

12 

15 

18 

21 

24 

27 

15 

1761 

1790 

1818 

1847 

1875 

1903 

1931 

1959 

1987 

2014 

3 

6 

8 

11 

14 

17 

20 

22 

25 

16 

2041 

2068 

2095 

2122 

2148 

2175 

2201 

2227 

2253 

2279 

3 

5 

8 

11 

13 

16 

18 

21 

24 

17 

2304 

2330 

2355 

2380 

2405 

2430 

2455 

2480 

2504 

2529 

2 

5 

7 

10 

12 

15 

17 

20 

22 

18 

2553 

2577 

2601 

2625 

2648 

2672 

2695 

2718 

2742 

2765 

2 

5 

7 

9 

12 

14 

16 

19 

21 

19 

2788 

2810 

2833 

2856 

2878 

2900 

2923 

2945 

2967 

2989 

2 

4 

7 

9 

11 

13 

16 

18 

20 

20 

3010 

3032 

3054 

3075 

3096 

3118 

3139 

3160 

3181 

3201 

2 

4 

6 

8 

11 

13 

15 

17 

19 

21 

3222 

3243 

3263 

3284 

3304 

3324 

3345 

3365 

3385 

3404 

2 

4 

6 

8 

10 

12 

14 

16 

18 

22 

3424 

3444 

3464 

3483 

3502 

3522 

3541 

3560 

3579 

3598 

2 

4 

6 

8 

10 

12 

14 

15 

17 

23 

3617 

3636 

3655 

3674 

3692 

3711 

3729 

3747 

3766 

3784 

2 

4 

6 

7 

9 

11 

13 

15 

17 

24 

3802 

3820 

3838 

3856 

3874 

3892 

3909 

3927 

3945 

3962 

2 

4 

5 

7 

g 

11 

12 

14 

16 

25 

3979 

3997 

4014 

4031 

4048 

4065 

4082 

4099 

4116 

4133 

2 

3 

5 

t^ 
l 

9 

10 

12 

14 

15 

26 

4150 

4166 

4183 

4200 

4216 

4232 

4249 

4265 

4281 

4298 

2 

3 

5 

t-i 
i 

8 

\Q 

11 

13 

15 

27 

4314 

4330 

4346 

4362 

4378 

4393 

4409 

4425 

4440 

4456 

2 

3 

5 

6 

8 

9 

11 

13 

14 

28 

4472 

4487 

4502 

4518 

4533 

4548 

4564 

4579 

4594 

4609 

2 

3 

5 

6 

8 

9 

11 

12 

14 

29 

4624 

4639 

4654 

4669 

4683 

4698 

4713 

4728 

4742 

4757 

1 

3 

4 

6 

7 

g 

10 

12 

13 

30 

4771 

4786 

4800 

4814 

4829 

4843 

4857 

4871 

4886 

4900 

3 

4 

6 

7 

9 

10 

11 

13 

31 

4914 

4928 

4942 

4955 

4969 

4983 

4997 

5011 

5024 

5038 

3 

4 

6 

7 

8 

10 

11 

12 

32 

5051 

5065 

5079 

5092 

5105 

5119 

5132 

5145 

5159 

5172 

3 

4 

C 

u 

7 

8 

9 

11 

12 

33 

5185 

5198 

5211 

5224 

5237 

5250 

5263 

5276 

5289 

5302 

3 

4 

c 

o 

6 

8 

9 

10 

12 

34 

5315 

5328 

5340 

5353 

5366 

5378 

5391 

5403 

5416 

5428 

3 

4 

c 

cJ 

6 

8 

9 

10 

11 

35 

5441 

5453 

5465 

5478 

5490 

5502 

5514 

5527 

5539 

5551 

1 

2 

4 

K 
O 

6 

7 

9 

10 

11 

36 

5563 

5575 

5587 

5599 

5611 

5623 

5635 

5647 

5658 

5670 

1 

2 

4 

5 

6 

7 

8 

10 

11 

37 

5682 

5694 

5705 

5717 

5729 

5740 

5752 

5763 

5775 

5786 

1 

2 

3 

5 

6 

7 

8 

9 

10 

38 

5798 

5809 

5821 

5832 

5843 

5855 

5866 

5877 

5888 

5899 

1 

2 

3 

5 

6 

7 

8 

9 

10 

39 

5911 

5922 

5933 

5944 

5955 

5966 

5977 

5988 

5999 

6010 

1 

2 

3 

4 

5 

7 

8 

9 

10 

40 

6021 

6031 

6042 

6053 

6064 

6075 

6085 

6096 

6107 

6117 

1 

2 

3 

4 

5 

6 

8 

9 

10 

41 

6128 

6138 

6149 

6160 

6170 

6180 

6191 

6201 

6212 

6222 

1 

2 

3 

4 

5 

6 

7 

8 

9 

42 

6232 

6243 

6253 

6263 

6274 

6284 

6294 

6304 

6314 

6325 

1 

2 

3 

4 

5 

6 

7 

8 

9 

43 

6335 

6345 

6355 

6365 

6375 

6385 

6395 

6405 

6415 

6425 

1 

2 

3 

4 

5 

6 

7 

8 

9 

44 

6435 

6444 

6454 

6464 

6474 

6484 

6493 

6503 

6513 

6522 

1 

2 

3 

4 

5 

6 

7 

8 

9 

45 

6532 

6542 

6551 

6561 

6571 

6580 

6590 

6599 

6609 

6618 

1 

2 

3 

4 

5 

6 

7 

8 

9 

46 

6628 

6637 

6646 

6656 

6665 

6675 

6684 

6693 

6702 

6712 

1 

2 

3 

4 

5 

6 

7 

7 

8 

47 

6721 

6730 

67396749 

6758 

6767 

6776 

6785 

6794 

6803 

1 

2 

3 

4 

5 

5 

6 

7 

8 

48 

6812 

6821 

6830 

6839 

6848 

6857 

6866 

6875 

6884 

6893 

1 

2 

3 

4 

4 

5 

6 

7 

8 

49 

6902 

6911 

6920 

6928 

6937 

6946 

6955 

6964 

6972 

6981 

1 

2 

3 

4 

4 

5 

6 

7 

8 

50 

6990 

6998 

7007 

7016 

7024 

7033 

7042 

7050 

7059 

7067 

1 

2 

3 

3 

4 

5 

6 

7 

8 

51 

7076 

7084 

7093 

7101 

7110 

7118 

7126 

7135 

7143 

7152 

1 

2 

3 

3 

4 

5 

6 

7 

8 

52 

7160 

7168 

7177 

7185 

7193 

7202 

7210 

7218 

7226 

7235 

1 

2 

2 

3 

4 

5 

6 

7 

7 

53 

7243 

7251 

7259 

7267 

7275 

7284 

7292 

7300 

7308 

7316 

1 

2 

2 

3 

4 

5 

6 

6 

7 

54 

7324 

7332 

7340 

7348 

7356 

7364 

7372 

7380 

7388 

7396 

1 

2 

2 

3 

4 

5 

6 

6 

7 

322 


LOGARITHMS  OF  NUMBERS. 


0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

PBOPOETIONAL  PARTS. 

1 

2 

3 

4 

5 

6 

7 

8 

9 

7404 

7412 

7419 

7427 

7435 

7443 

7451 

7459 

7466 

7474 

2 

2 

3 

4 

5 

c 
tj 

6 

7 

7482 

74907497 

7505 

7513 

7520 

7528 

7536 

7543 

7551 

2 

2 

3 

4 

5 

r 

u 

6 

7 

7559 

7566 

7574 

7582 

7589 

7597 

7604 

7612 

7619 

7627 

2 

2 

3 

4 

5 

5 

6 

7 

7634 

7642 

7649 

7657 

7664 

7672 

7679 

7686 

7694 

7701 

2 

3 

4 

4 

5 

6 

7 

7709 

7716 

7723 

7731 

7738 

7745 

7752 

7760 

7767 

7774 

2 

3 

4 

4 

5 

6 

7 

7782 

7789 

7796 

7803 

7810 

7818 

7825 

7832 

7839 

7846 

2 

3 

4 

4 

5 

6 

6 

7853 

7860 

7868 

7875 

7882 

7889 

7896 

7903 

7910 

7917 

2 

3 

4 

4 

5 

6 

6 

7924 

7931 

7938 

7945 

7952 

7959 

7966 

7973 

7980 

7987 

2 

3 

3 

4 

5 

6 

6 

7993 

8000 

8007 

8014 

8021 

8028 

8035 

8041 

8048 

8055 

2 

3 

3 

4 

5 

6 

8062 

8069 

8075 

8082 

8089 

8096 

8102 

8109 

8116 

8122 

2 

3 

3 

4 

5 

6 

8129 

8136 

8142 

8149 

8156 

8162 

8169 

8176 

8182 

8189 

2 

3 

3 

4 

5 

6 

8195 

82C2  8209 

8215 

8222 

8228 

8235 

8241 

8248 

8254 

2 

3 

3 

4 

5 

6 

8261 

8267 

8274 

8280 

8287 

8293 

8299 

8306 

8312 

8319 

2 

3 

3 

4 

5 

6 

8325 

8331 

8338 

8344 

8351 

8357 

8363 

8370 

8376 

8382 

2 

3 

3 

4 

4 

6 

8388 

8395 

8401 

8407 

8414 

8420 

8426 

8432 

8439 

8445 

1 

2 

2 

3 

4 

4 

6 

8451 

8457 

8463 

8470 

8476 

8482 

8488 

8494 

8500 

8506 

1 

2 

2 

3 

4 

4 

6 

8513 

8519 

8525 

8531 

8537 

8543 

8549 

8555 

8561 

8567 

1 

2 

2 

3 

4 

4 

5 

8573 

8579 

8585 

8591 

8597 

8603 

8609 

8615 

8621 

8627 

1 

2 

2 

3 

4 

4 

5 

8633 

8639 

8645 

8651 

8657 

8663 

8669 

8675 

8681 

8686 

1 

2 

2 

3 

4 

4 

5 

8692 

8698 

8704 

8710 

8716 

8722 

8727 

8733 

8739 

8745 

1 

2 

2 

3 

4 

4 

5 

8751 

8756 

8762 

8768 

8774 

8779 

8785 

8791 

8797 

8802 

1 

2 

2 

3 

3 

4 

5 

8808 

8814 

8820 

8825 

8831 

8837 

8842 

8848 

8854 

8859 

1 

2 

2 

3 

3 

4 

5 

8865|8871 

8876 

8882 

8887 

8893 

8899 

8904 

8910 

8915 

1 

2 

2 

3 

3 

4 

4 

5 

8921 

8927 

8932 

8938 

8943 

8949 

8954 

8960 

8965 

8971 

1 

2 

2 

3 

3 

4 

4 

5 

8976 

8982 

8987 

8993 

8998 

9004 

9009 

9015 

9020 

9026 

1 

1 

2 

2 

3 

3 

4 

4 

5 

9031 

9036 

9042 

9047 

9053 

9058 

9063 

9069 

9074 

9079 

1 

1 

2 

2 

3 

3 

4 

4 

5 

9085 

9090 

9096 

9101 

9106 

9112 

9117 

9122 

9128 

9133 

1 

1 

2 

2 

3 

3 

4 

4 

5 

9138 

9143 

9149 

9154 

9159 

9165 

9170 

9175 

9180 

9186 

1 

1 

2 

2 

3 

3 

4 

4 

5 

9191 

9196 

9201 

9206 

9212 

9217 

9222 

9227 

9232 

9238 

1 

1 

2 

2 

3 

3 

4 

4 

5 

9243 

9248 

9253 

9258 

9263 

9269 

9274 

9279 

9284 

9289 

1 

1 

2 

2 

3 

3 

4 

4 

5 

9294 

9299 

9304 

9309 

9315 

9320 

9325 

9330 

9335 

9340 

1 

1 

2 

2 

3 

3 

4 

4 

5 

9345 

9350 

9355 

9360 

9365 

9370 

9375 

9380 

9385 

9390 

1 

1 

2 

2 

3 

3 

4 

4 

5 

9395 

94009405 

9410 

9415 

9420 

9425 

9430 

9435 

9440 

0 

1 

1 

2 

2 

3 

3 

4 

4 

9445 

94509455 

9460 

9465 

9469 

9474 

9479 

9484 

9489 

0 

1 

1 

2 

2 

3 

3 

4 

4 

9494 

9499 

9504 

9509 

9513 

9518 

9523 

9528 

9533 

9538 

0 

1 

1 

2 

2 

3 

3 

4 

4 

9542 

9547 

9552 

9557 

9562 

9566 

9571 

9576 

9581 

9586 

0 

1 

2 

2 

3 

3 

4 

4 

9590  9595i  9600  9605 
9638964396479652 

9609 
9657 

9614 
9661 

9619 
9666 

9624 
9671 

9628 
9675 

9633 
9680 

0 
0 

1 
1 

2 
2 

2 
2 

3 

3 

3 
3 

4 
4 

4 
4 

968519689  9694 

9699 

9703 

9708 

9713 

9717 

9722 

9727 

0 

1 

2 

2 

3 

3 

4 

4 

9731 

9736 

9741 

9745 

9750 

9754 

9759 

9763 

9768 

9773 

0 

1 

2 

2 

3 

3 

4 

4 

9777 

9782 

9787 

9791 

9795 

9800 

9805 

9809 

9814 

9818 

0 

1 

2 

2 

3 

3 

4 

4 

9823  9827  9832 

9836 

9841 

9845 

9850 

9854 

9859 

9863 

0 

1 

2 

2 

3 

3 

4 

4 

9868  9872  9877 

9881 

9886 

9890 

9894 

9899 

9903 

9908 

0 

1 

2 

2 

3 

3 

4 

4 

991299179921 

9926 

9930 

9934 

9939 

9943 

9948 

9952 

0 

1 

2 

2 

3 

3 

4 

4 

9956  9961  9965 

9969 

9974 

9978 

9983 

9987 

9991 

9996 

0 

1 

2 

2 

3 

3 

3 

4 

323 


ANTILOGARITHMS. 


Loga-  j 
rithnis.  | 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

PROPORTIONAL  PARTS. 

2 

9 

.00 

1000 

1002 

1005 

1007 

1009 

1012 

1014 

1016 

1019 

1021 

0 

0 

1 

1 

1 

2 

2 

.01 

1023 

1026 

1028 

1030 

1033 

1035 

1038 

1040 

1042 

1045 

0 

0 

1 

1 

1 

2 

2 

2 

.02 

1047 

1050 

1052 

1054 

1057 

1059 

1062 

1064 

1067 

1069 

0 

0 

1 

1 

1 

2 

2 

2 

.03 

1072 

1074 

1076 

1079 

1081 

1084 

1086 

1089 

1091 

1094 

0 

0 

1 

1 

1 

2 

2 

2 

.04 

1096 

1099 

1102 

1104 

1107 

1109 

1112 

1114 

1117 

1119 

0 

1 

1 

1 

2 

2 

2 

2 

.05 

1122 

1125 

1127 

1130 

1132 

1135 

1138 

1140 

1143 

1146 

0 

1 

1 

1 

1 

2 

2 

2 

2 

.06 

1148 

1151 

1153 

1156 

1159 

1161 

1164 

1167 

1169 

1172 

0 

1 

1 

1 

1 

2 

2 

2 

2 

.07 

1175 

1178 

1180 

1183 

1186 

1189 

1191 

1194 

1197 

1199 

0 

1 

I 

1 

1 

2 

2 

2 

2 

.08 

1202 

1205 

1208 

1211 

1213 

1216 

1219 

1222 

1225 

1227 

0 

1 

1 

1 

2 

2 

2 

3 

.09 

1230 

1233 

1236 

1239 

1242 

1245 

1247 

1250 

1253 

1256 

0 

1 

1 

1 

2 

2 

2 

3 

.10 

1259 

1262 

1265 

1268 

1271 

1274 

1276 

1279 

1282 

1285 

0 

1 

1 

2 

2 

2 

3 

.11 

1288 

1291 

1294 

1297 

1300 

1303 

1306 

1309 

1312 

1315 

0 

1 

2 

2 

2 

2 

3 

.12 

1318 

1321 

1324 

1327 

1330 

1334 

1337 

1340 

1343 

1346 

0 

1 

2 

2 

2 

2 

3 

.13 

1349 

1352 

1355 

1358 

1361 

1365 

1368 

1371 

1374 

1377 

0 

1 

2 

2 

2 

3 

3 

.14 

1380 

1384 

1387 

1390 

1393 

1396 

1400 

1403 

1406 

1409 

0 

1 

2 

2 

2 

3 

3 

.15 

1413 

1416 

1419 

1422 

1426 

1429 

1432 

1435 

1439 

1442 

0 

1 

1 

% 

2 

2 

3 

3 

.16 

1445 

1449 

1452 

1455 

1459 

1462 

1466 

1469 

1472 

1476 

0 

1 

1 

2 

2 

2 

3 

3 

.17 

1479 

1483 

1486 

1489 

1493 

1496 

1500 

1503 

1507 

1510 

0 

1 

2 

2 

2 

3 

3 

.18 

1514 

1517 

1521 

1524 

1528 

1531 

1535 

1538 

1542 

1545 

0 

1 

2 

2 

2 

3 

3 

.19 

1549 

1552 

1556 

1560 

1563 

1567 

1570 

1574 

1578 

1581 

0 

1 

2 

2 

3 

3 

3 

.20 

1585 

1589 

1592 

1596 

1600 

1603 

1607 

1611 

1614 

1618 

0 

1 

1 

2 

2 

3 

3 

3 

.21 

1622 

1626 

1629 

1633 

1637 

1641 

1644 

1648 

1652 

1656 

0 

1 

2 

2 

2 

3 

3 

3 

.22 

1660 

1663 

1667 

1671 

1675 

1679 

1683 

1687 

1690 

1694 

0 

1 

2 

2 

2 

3 

3 

3 

.23 

1698 

1702 

1706 

1710 

1714 

1718 

1722 

1726 

1730 

1734 

0 

1 

2 

2 

2 

3 

3 

4 

.24 

1738 

1742 

1746 

1750 

1754 

1758 

1762 

1766 

1770 

1774 

0 

1 

2 

2 

2 

3 

3 

4 

.25 

1778 

1782 

1786 

1791 

1795 

1799 

1803 

1807 

1811 

1816 

0 

1 

2 

2 

2 

3 

3 

4 

.26 

1820 

1824 

1828 

1832 

1837 

1841 

1845 

1849 

1854 

1858 

0 

1 

2 

2 

3 

3 

3 

4 

.27 

1862 

1866 

1871 

1875 

1879 

1884 

1888 

1892 

1897 

1901 

0 

1 

2 

2 

3 

3 

3 

4 

.28 

1905 

1910 

1914 

1919 

1923 

1928 

1932 

1936 

1941 

1945 

0 

1 

2 

2 

3 

3 

4 

4 

.29 

1950 

1954 

1959 

1963 

1968 

1972 

1977 

1982 

1986 

1991 

0 

1 

2 

2 

3 

3 

4 

4 

.30 

1995 

2000 

2004 

2009 

2014 

2018 

2023 

2028 

2032 

2037 

0 

1 

2 

2 

3 

3 

4 

4 

.31 

2042 

2046 

2051 

2056 

2061 

2065 

2070 

2075 

2080 

2084 

0 

1 

2 

2 

3 

3 

4 

4 

.32 

2089 

2094 

2099 

2104 

2109 

2113 

2118 

2123 

2128 

2133 

0 

2 

2 

3 

3 

4 

4 

.33 

2138 

2143 

2148 

2153 

2158 

2163 

2168 

2173 

2178 

2183 

0 

2 

2 

3 

3 

4 

4 

.34 

2188 

2193 

2198 

2203 

2208 

2213 

2218 

2223 

2228 

2234 

1 

2 

2 

3 

3 

4 

4 

5 

.35 

2239 

2244 

2249 

2254 

2259 

2265 

2270 

2275 

2280 

2286 

1 

2 

2 

3 

3 

4 

4 

5 

.36 

2291 

2296 

2301 

2307 

2312 

2317 

2323 

2328 

2333 

2339 

1 

2 

2 

3 

3 

4 

4 

5 

.37 

2344 

2350 

2355 

2360 

2366 

2371 

2377 

2382 

2388 

2393 

1 

2 

2 

3 

3 

4 

4 

5 

.38 

2399 

2404 

2410 

2415 

2421 

2427 

2432 

2438 

2443 

2449 

2 

2 

3 

3 

4 

4 

5 

.39 

2455 

2460 

2466 

2472 

2477 

2483 

2489 

2495 

2500 

2506 

2 

2 

3 

3 

4 

5 

5 

.40 

2512 

2518 

2523 

2529 

2535 

2541 

2547 

2553 

2559 

2564 

2 

2 

3 

4 

4 

5 

5 

,41 

2570 

2576 

2582 

2588 

2594 

2600 

2606 

2612 

2618 

2624 

2 

2 

3 

4 

4 

5 

5 

.42 

2630 

2636 

2642 

2649 

2655 

2661 

2667 

2673 

2679 

2685 

2 

2 

3 

4 

4 

5 

6 

.43 

2692 

2698 

2704 

2710 

2716 

2723 

2729 

2735 

2742 

2748 

2 

3 

3 

4 

4 

5 

6 

.44 

2754 

2761 

2767 

2773 

2780 

2786 

2793 

2799 

2805 

2812 

2 

3 

3 

4 

4 

5 

6 

.45 

2818 

2825 

2831 

2838 

2844 

2851 

2858 

2864 

2871 

2877 

1 

2 

3 

3 

4 

5 

5 

6 

.46 

2884 

2891 

2897 

2904 

2911 

2917 

2924 

2931 

2938 

2944 

1 

2 

3 

3 

4 

5 

5 

6 

.47 

29512958 

2965 

2972 

2979 

2985 

2992  i  2999  3006 

3013 

1 

2 

3 

3 

4 

5 

5 

6 

.43 
.49 

30203027 
30903097 

3034 
3105 

3041 
3112 

3048 
3119 

3055 
3126 

3062 
3133 

3069 
3141 

3076 
3148 

3083 
3155 

1 
1 

2 
2 

3 
3 

4 
4 

4 
4 

5 
5 

6 
6 

6 

a 

324 


ANTILOGAKITHMS. 


&i 

3'Z 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

PROPORTIONAL  PARTS. 

*—  i 

3228 

1 

2 

1 

2 

3 

4 

4 

5 

6 

7 

.50 

162 

3170 

3177 

3184 

3192 

199 

3206 

3214 

3221 

.51 

236 

3243 

3251 

3258 

3266 

273 

3281 

3289 

3296 

3304 

2 

2 

3 

4 

5 

5 

6 

7 

.52 

311 

3319 

3327 

3334 

3342 

3350 

3357 

3365 

3373 

3381 

2 

2 

3 

4 

5 

5 

6!  7 

.53 

388 

3396 

3404 

3412 

3420 

3428 

3436 

3443 

3451 

3459 

2 

2 

3 

4 

5 

6 

6  7 

.54 

467 

3475 

3483 

3491 

3499 

508 

3516 

3524 

3532 

3540 

2 

2 

3 

4 

5 

6 

6 

7 

.55 

548 

3556 

3565 

3573 

3581 

589 

3597 

3606 

3614 

3622 

2 

2 

3 

4 

5 

6 

7 

7 

.56 

631 

3639 

3648 

3656 

3664 

673 

3681 

3690 

3698 

3707 

2 

3 

3 

4 

5 

6 

7 

8 

.57 

715 

3724 

3733 

3741 

3750 

758 

3767 

3776 

3784 

3793 

2 

3 

3 

4 

5 

6 

7 

8 

.58 

802 

3811 

3819 

3828 

3837 

3846 

3855 

3864 

3873 

3882 

2 

3 

4 

4 

5 

6 

7 

8 

.59 

890 

3899 

3908 

3917 

3926 

936 

3945 

3954 

3963 

3972 

2 

3 

4 

5 

5 

6 

7 

8 

.60 

981 

3990 

999 

4009 

4018 

4027 

4036 

4046 

4055 

4064 

1 

2 

3 

4 

5 

6 

6 

7 

8 

.61 

4074 

4083 

093 

4102 

4111 

121 

4130 

4140 

4150 

4159 

1 

2 

3 

4 

5 

6 

7 

8 

9 

62 

169 

4178 

188 

4198 

4207 

4217 

4227 

4236 

4246 

4256 

1 

2 

3 

4 

5 

6 

7 

8 

9 

.63 

4266 

4276 

4285 

4295 

4305 

315 

4325 

4335 

4345 

4355 

1 

2 

3 

4 

5 

6 

7 

8 

9 

64 

365 

4375 

4385 

4395 

4406 

416 

4426 

4436 

4446 

4457 

1 

2 

3 

4 

5 

6 

7 

8 

9 

.65 

4467 

4477 

4487 

4498 

4508 

4519 

4529 

4539 

4550 

4560 

1 

2 

3 

4 

5 

6 

7 

8 

9 

.66 

571 

4581 

4592 

4603 

4613 

4624 

4634 

4645 

4656 

4667 

1 

2 

3 

4 

5 

6 

7 

9 

10 

.67 

4677 

4688 

4699 

4710 

4721 

4732 

4742 

4753 

4764 

4775 

1 

2 

3 

4 

5 

7 

8 

9 

10 

.68 

786 

4797 

4808 

4819 

4831 

4842 

4853 

4864 

4875 

4887 

1 

2 

3 

4 

6 

7 

8 

9 

10 

.69 

4898 

4909 

4920 

4932 

4943 

4955 

4966 

4977 

4989 

5000 

1 

2 

3 

5 

6 

7 

8 

9 

10 

.70 

5012 

5023 

5035 

5047 

5058 

5070 

5082 

5093 

5105 

5117 

1 

2 

4 

5 

6 

7 

8 

9 

11 

.71 

5129 

5140 

5152 

5164 

5176 

5188 

5200 

5212 

5224 

5236 

1 

2 

4 

5 

6 

7 

8 

10 

11 

.72 

5248 

5260 

5272 

5284 

5297 

5309 

5321 

5333 

5346 

5358 

1 

2 

4 

5 

6 

7 

9 

10 

11 

.73 

5370 

5383 

5395 

5408 

5420 

5433 

5445 

5458 

5470 

5483 

1 

3 

4 

5 

6 

8 

9 

10 

11 

.74 

5495 

5508 

5521 

5534 

5546 

5559 

5572 

5585 

5598 

5610 

1 

3 

4 

5 

6 

8 

9 

10 

12 

.75 

5623 

5636 

5649 

5662 

5675 

5689 

5702 

5715 

5728 

5741 

1 

3 

4 

5 

7 

8 

9 

10 

12 

.76 

5754 

5768 

5781 

5794 

5808 

5821 

5834 

5848 

5861 

5875 

1 

3 

4 

5 

7 

8 

9 

1 

12 

.77 

5888 

5902 

5916 

5929 

5943 

5957 

5970 

5984 

5998 

6012 

1 

3 

4 

5 

7 

8 

10 

11 

12 

.78 

6026 

6039 

6053 

6067 

6081 

6095 

6109 

6124 

6138 

6152 

1 

3 

4 

6 

7 

8 

10 

11 

13 

.79 

6166 

6180 

6194 

6209 

6223 

6237 

6252 

6266 

6281 

6295 

1 

3 

4 

6 

7 

9 

10 

11 

13 

.80 

6310 

6324 

6339 

6353 

6368 

6383 

6397 

6412 

6427 

6442 

1 

3 

4 

6 

7 

9 

10 

12 

13 

.81 

6457 

6471 

6486 

6501 

6516 

6531 

6546 

6561 

6577 

6592 

2 

3 

5 

6 

8 

9 

11 

12 

14 

.82 

6607 

6622 

6637 

6653 

6668 

6683 

6699 

6714 

6730 

6745 

2 

3 

5 

6 

8 

9 

11 

12 

14 

.83 

6761 

6776 

6792 

6808 

6823 

6839 

6855 

6871 

6887 

6902 

2 

3 

5 

6 

8 

9 

11 

13 

14 

.84 

6918 

6934 

6950 

6966 

6982 

6998 

7015 

7031 

7047 

7063 

2 

3 

5 

6 

8 

10 

11 

13 

15 

.85 

7079 

7096 

7112 

7129 

7145 

7161 

7178 

7194 

7211 

7228 

2 

3 

5 

7 

8 

10 

12 

13 

15 

.86 

7244 

7261 

7278 

7295 

7311 

7328 

7345)7362 

7379 

7396 

2 

3 

5 

7 

8 

10 

12 

13 

15 

.87 

7413 

7430 

7447 

7464 

7482 

7499 

7516,7534 

7551 

7568 

2 

3 

5 

7 

9 

10 

12 

14|16 

.88 

7586 

7603 

7621 

7638 

7656 

7674 

769117709 

7727 

7745 

2 

4 

5 

7 

9 

11 

12 

14 

16 

.89 

7762 

7780 

7798 

7816 

7834 

7852 

7870 

7889 

7907 

7925 

2 

4 

5 

7 

9 

11 

13 

14 

16 

.90 

7943 

7962 

7980 

7998 

8017 

8035 

8054 

i8072 

18091 

8110 

2 

4 

6 

7 

9 

11 

13 

15 

17 

.91 

8128 

8147 

8166 

8185 

8204 

8222 

8241  8260}8279 

8299 

2 

4 

6 

8 

9 

11 

13 

15 

17 

.92 

8318 

8337 

8356 

8375 

8395 

8414 

8433  8453 

8472 

8492 

2 

4 

6 

8 

10 

12 

14 

15 

17 

.93 

8511 

8531 

8551 

8570 

8590 

8610 

8630 

8650 

8670 

8690 

2 

4 

6 

8 

10 

12 

14 

16 

18 

.94 

871C 

8730 

8750 

877C 

8790 

8810 

8831 

8851 

8872 

8892 

2 

4 

6 

8 

10 

12 

14 

16 

18 

.95 

89K 

8933 

8954 

'8974 

8995 

9016 

9036 

9057 

9078 

9099 

2 

4 

6 

8 

10 

12 

15 

17 

19 

.96 

912C 

9141 

9162 

9182 

9204 

9226 

9247 

9268 

929C 

9311 

2 

4 

6 

8 

11 

13 

15 

17 

19 

.97 

933C 

9354 

937e 

>9397 

9419 

9441 

9462 

9484 

9506 

9528 

2 

4 

7 

9 

11 

13 

1  t 

17 

20 

.98 

955C 

9572 

9594 

9616 

9638 

9661 

9683 

9705 

9727 

9750 

2 

4 

7 

9 

11 

13 

16 

18 

20 

.99 

9775 

979£ 

9817 

984C 

9863 

9886 

9908 

9931 

9954 

9977 

2 

5 

7 

9 

1 

14 

16 

18 

20 

1 

325 


INDEX 


A 

PAGE 

Acetaldehyde,  formation  from  lactic  acid 50 

use  in  cadmium  determination 179 

Acetic  acid,  reduction  of 50 

Accumulator  cell,  single,  use  in  constant  potential  work  118, 120, 130, 145,229 

Acetone,  use  in  thallium  determination 203 

Alcohol,  use  for  reduction  of  peroxides 131 

Alkali  metals,  deposition  as  amalgams 214,  217,  218 

separation  from  aluminium  and  iron .  .  . 220 

cadmium 259 

calcium 219 

copper  235 

lead    284 

magnesium  and  heavy  metals 218 

mercury 251 

molybdenum 207 

Alkaline  earths,  deposition  as  amalgams 218 

separation  from  aluminium  and  iron 220 

Alkali  solution,  standardization  of    224 

Aluminium,  deposition  as  hydroxide 209 

separation  from  alkalies  and  alkaline  earths 220 

beryllium 273 

cadmium 259 

chromium 271 

cobalt 281 

copper 235 

iron 269, 270, 271, 273, 274 

lead  284 

mercury 250 

nickel  and  uranium 280 

silver • 244 

zinc 258 

Amalgamation  of  brass  wire  cathodes 178 

Amalgams,  determination  of  metals  as 49,  80,  104,  205,  214 

Amberg,  deposition  of  palladium  with  stirring 61 

Ammoniacal  solutions  as  complex  electrolytes 130 

Ammonium,  indirect  determination  of 221 

Ammonium  nitrate,  formation  from  nitric  acid 48 

Ampere 7, 11 

Amperemeter  or  ammeter 8 

Aniline,  formed  by  electrolysis 49,  50 

Anion    4 

327 


328  INDEX 

PAGK 

Anode 3, 4 

Anodes,  shape  of ^. • 54, 57 

Anodes  of  passive  iron 187 

Antimonial  lead 301 

Antimony 153 

behavior  toward  roughened  platinum  dishes 59 

commercial 316 

deposition  from  sodium  sulphide  solution 153, 157 

deposition  in  the  presence  of  lead  sulphate 289 

separation  from  alkalies  and  alkaline  earths 219 

arsenic 153, 255 

bismuth 257 

cadmium 259 

copper 130,  158,  236 

lead 284 

silver 245 

tin 153, 253, 255, 256, 300 

solution  in  polysulphides 153, 314 

Antimony-lead-tin-copper  alloys 300 

Apparatus  for  deposition  at  definite  cathode  potential *.     148 

rapid  electroanalysis 64, 73 

Arrhenius,  theory  of  electrolytic  dissociation 4 

Arsenic 162 

behavior  in  the  presence  of  ferric  sulphate 234, 289 

detection  of 162 

separation  from  antimony 153, 295 

cadmium , 255 

copper 130,233,234 

lead 196, 308, 309 

mercury 250 

silver 246 

tin 256 

volatilization  as  trichloride 304 

Ashcroft,  magnetic  stirring 73 

Atomic  weights,  international  table  of 320 

Auxiliary  electrodes 40, 148, 150, 170, 227 

B 

Barium  chloride,  determination  of  Ba  and  Cl  in 219 

deposition  as  amalgam 209 

separation  by  potential  difference 209 

from  aluminium,  iron,  calcium,  and  magnesium      219,  220 

cadmium 259 

calcium,  magnesium  and  heavy  metals 218 

copper 235 

lead 284 

mercury 251 

uranium .  .  221 


INDEX  329 

PAGB 

Bath  potential,  measurement  of 34,  146 

Bearing  metal 301,  302 

Beryllium,  separation  from  aluminium 273 

iron 273 

lead 284 

Bicarbonates,  formation  from  oxajates 50 

Bismuth,  behavior  in  presence  of  lead  sulphate 236,  288 

deposition  from  nitric  acid  solutions 145 

separation  from  antimony 257 

cadmium 260 

copper 130,  235 

lead 196 

mercury 252 

silver 248 

zinc 259 

Black  copper  deposits 233 

Blue  powder 305 

Bone  A.  J.,  apparatus  for  rapid  electrolysis 73 

Brass,  analysis  of 296 

deposition  from  potassium  cyanide  solution 93 

Brass  gauze  electrodes 178 

Britannia  metal 300 

Bromine,  determination  in  the  presence  of  potassium 217 

separation  from  chlorine 213 

iodine , 213 

Bronzes 298,  300 

C 

Cadmium,  decomposition  potential  in  potassium  cyanide  solutions  at 

different  temperatures 94 

deposition  after  copper  in  acid  solutions 93 

before  copper  in  potassium  cyanide  solution ...    93,  96 

from  oxalate  solutions 178 

other  solutions 179 

potassium  cyanide  solution 176,  177,  228 

sulphuric  acid  solution 81,  91,  174 

possibility  of  separation  from  acid  solutions 91,  174 

separation  from  alkalies  and  alkaline  earths  219,  259 

aluminium,  antimony  and  arsenic 259 

bismuth 260 

cobalt 260 

copper 120,  130,  228,  229 

iron 261 

lead 284 

manganese .    262 

mercury . 251,  262 

nickel 262 

silver 263 

zinc .  .  264 


330  INDEX 

PAGE 

Calcium,  behavior  as  amalgam  in  presence  of  magnesium 219, 220 

deposition  as  amalgam 209 

separation  by  graded  potential 209 

separation  from  alkalies 219 

barium  and  strontium 219 

cadmium 259 

copper 235 

lead 284 

magnesium  and  heavy  metals 218,  220 

mercury 251 

titration  of  calcium  hydroxide 220 

Canarin,  formation  from  ammonium  thiocyanate 140 

Capillary  electrometer 43,  149,  150 

Carbon,  deposition  from  oxalates,  citrates  and  tartrates 181 

Carbon  dioxide,  formation  from  organic  acids 50 

Carbonic  acid,  determination  in  the  presence  of  sodium 218 

Caspari,  overvoltage  of  hydrogen 174 

Cathode  potential. 64,  66,  148,  227 

Cathodes 3,  4,  54,  59,  65 

Cation *        4 

Cerium,  separation  from  alkalies  and  alkaline  earths 219 

iron    274 

Chloride  of  potassium,  determination  of  potassium  and  chlorine  in 217 

Chlorides  and  hydrochloric  acid,  behavior  toward  the  current 5,  48 

Chlorine,  separation  from  bromine 213 

iodine 213 

Chrome-alum,  use  in  manganese  determination 197 

Chrome-nickel  steel 314 

Chromium,  deposition  as  amalgam 205 

determination  as  lead  chromate 3 14 

oxidation  to  chromate    205 

separation  from  alkalies  and  alkaline  earths 218 

aluminium,  iron  and  uranium .  .  .      270,  271,  272 

cobalt 281 

copper 236 

lead 284 

nickel 279 

Cinnabar , 318 

Citrate  solutions,  deposition  of  carbon  from 182 

Clarke,  separation  of  antimony  and  tin 301 

Classen,  electrolysis  of  oxalates 50 

Cobalt,  behavior  toward  potassium  cyanide 263 

deposits  with  carbon  content 191 

determination 191 

separation  from  alkalies  and  alkaline  earths 218,  219 

aluminium,  chromium,  uranium  and  nickel.  . .  .     280 

cadmium 260 

copper   240 


INDEX  331 

PAGE 

Cobalt,  separation  from  iron 265 

lead 284 

mercury 251 

zinc 280 

Coffetti  and  Foerster,  decomposition  potentials 90 

Commercial  copper 287 

crude  lead 310 

zinc 303 

Compensation  method 37 

Complete  deposition  of  a  metal 90,  132 

Complex  cyanides 51 

electrolytes 50,  138,  153,  185 

Complexity,  different  degrees  of 95 

Computations,  electrochemical  1 1,  12 

Concentration,  cells , 39 

definition  of 26 

determination  of  by  conductivity  measurements    99 

Conductance 20 

equivalent 21 

determination  of  concentration 99 

measurement  for  recognition  of  complexity 176 

specific 20 

unit  of 20 

Conductance  salts 17 

Conductors,  good  and  bad 2,  3 

of  the  first  class 7 

of  the  second  class 7 

Constant  cathode  potential,  deposition  at 148 

current  strength,  deposition  at 116,  117 

potential,  use  of  storage  cells  for 118,  120,  127,  130,  145,  229 

Converter  copper   294 

Copper 116,  225 

added  to  bath  to  prevent  evolution  of  hydrogen    197 

chloride,  electrolysis  of 49 

commercial 287 

deposition  from  ammoniacal  solution 129 

nitric  acid  solution 124 

sulphuric  acid  solution 47,  116,  121 

rapid  deposition  from  nitric  acid  solution 128 

sulphuric  acid  solution 121 

with  magnetic  stirring 75,  77 

separation  from  aluminium,  magnesium,  alkalies  and  alkaline 

earths 235 

antimony  '. 129,  236 

arsenic 233,  234 

bismuth    235 

cadmium 93,  95,  96,  228,  229 

chlorine,  zinc,  arsenic,  antimony,  lead,  bismuth      130 


332  INDEX 

PAOB 

Copper,  separation  from  mercury,  cadmium  and  nickel 130 

chromium 236 

cobalt,  nickel 240 

iron 127,  237,  238,  292 

magnesium  and  manganese 239 

mercury 230 

molybdenum,  tungsten 241 

nickel 240 

nickel,  cadmium  and  zinc  in  sulphuric  acid 

solution 120 

palladium .  .  . 242 

platinum 242 

selenium  and  tellurium 242 

silver 225 

tin 243 

uranium 243 

zinc 78,  243,  296 

Copper-lead-tin-antimony  alloys 300 

manganese t    317 

matte 295,  297 

ores 295 

salts,  behavior  toward  the  current 47 

complex  cyanides  of 51,  95 

decomposition  potential  of 94 

dissociation  of  simple  and  complex  salts 130 

slags 294 

Coulomb 11 

Crucible  cathode,  rotating 65 

Crude  lead 310 

Current,  action  upon  simple  and  complex  electrolytes . 44,  45,  50 

density,  in  stirred  electrolytes 64 

normal,  NDioo 122 

significance  of 87,  88,  131 

its  role  in  electrolysis 3 

lines 86 

origin  of 27 

strength K 7,  8 

yield 13 

Cyanides,  complex 51 

D 

Daniell  cell 26 

Danneel,  deposition  of  metals 83 

Decomposition  potentials  at  different  current  densities  and  tempera- 
tures      93,276 

in  complex  electrolytes 92 

increase  during  decomposition 86 


INDEX  333 

PAGE 

^Decomposition  potentials  in  simple  electrolytes 31,  81,  91 

values 26,  32,  33,  80,  81 

Depolarizer. " 280,  282 

Deposition  at  constant  cathode  potential 147,  151 

current  strength 116 

voltage 118 

of  the  last  traces  of  a  metal 90,  132 

Deposits,  character  of 52,  81,  85,  88 

properties,  metallic  and  oxidic 1,  2,  79 

washing,  drying  and  weighing 120 

Determinations,  electroanalytical 113 

Deviations,  periodic  in  the  voltage 157 

Diaphragms,  use  in  antimony  determination 153 

Diffusion  in  electrolytes 60,  64,  87,  118 

Discharge  potential 88 

Dish  electrodes 54,  57,  59,  156,  194 

Disk  anodes 56,  57 

Dissociation,  electrolytic 4 

incomplete 6 

of  complex  KAu(CN)4 138 

Ni(NH3)4 185 

Double  layer,  electrical 25 

Drop  electrode 39 

Drying  of  deposits 120 

Duration  of  electrolysis 52 

according  to  Faraday's  Law 9,  185 

and  rate  of  stirring 61 

E 

Electrical  double  layer 25 

Electricity,  unit  of  quantity 11 

Electric  motor  for  rapid  electro-analysis 67,  68,  70 

Electro-analysis 3 

Electro-analytical  apparatus 40,  43,  53,  64,  73,  148 

Electrode-potential  in  separation    64,  66,  81,  147,  227 

Electrodes 2 

gauze 59 

rotating 57,  65,  67 

shape  of 53-67 

Electrolysis 3 

duration  of 52 

Electrolytes 2 

behavior  toward  the  current 44,  47 

complex 50 

simple 45,  83 

stirred  58, 60 

Electrolytic  dissociation,  theory 4 

of  complex  electrolytes 51 


334  INDEX 

PAGE 

Electrolytic  solution  pressure 24 

stands 54,  55,  56,  57,  68,  70 

Electrometric  titrations 99 

Electromotive  force,  formation  of 23 

measurement  of 37 

of  a  galvanic  element 28 

of  polarization 31 

opposing  force 23,  29 

potential  or  voltage 21 

significance  of,  in  electrolysis 84 

unit  of   8 

Electrons 108 

Electron  theory 109 

End  of  electrolysis,  chemical  test  for 120 

danger  of  overstepping  in  iron  determination 182 

test  by  means  of  test  electrode 135 

test  by  raising  level  of  electrolyte 125,  194 

Equivalent  conductance 21 

weights ' 9 

Ether  formation  from  organic  acids .• t  50 

Ethylene,  formation  from  organic  acids 50 

F 

Fairlie  and  Bone,  electrolytic  outfit 73 

Faraday 4 

unit 11 

Faraday's  law 9,  10,  13,  53,  61,  62,  63 

nomenclature 4 

Ferro-  and  ferricyanides,  determination  in  presence  of  potassium 217 

Fischer's  rotating  cathode 65,  66 

gauze  electrodes  with  stirrer 65,  67 

Flue  dust,  determination  of  zinc  in 305 

Foerster,  and  Cofietti,  decomposition  potentials 90 

deposition  of  copper  from  sulphuric  acid  solutions 119 

effect  of  temperature  on  complex  electrolytes 93 

Formaldehyde,  use  in  cadmium  determination 179 

Formation  of  electromotive  force 23 

Frary,  magnetic  stirring 74 

Friction 16 

G 

Gauze  electrodes 59,  65,  67,  157 

German  silver 240,  277 

Glycollic  acid,  formation  from  oxalic  acid 190 

Gold,  deposition  from  ammonium  thiocyanate  solution 140 

potassium  cyanide  solution 138,  139 

sodium  sulphide  solution 139 

separation  from  palladium 253 


INDEX  335 

PAGE 

Gold,  separation  from  platinum 252 

removal  of  the  deposit 140 

Gooch  and  Medway,  rotating  crucible  electrode .       65 

Gulcher  thermopile 131,  212 

H 

Halogens,  deposition  as  silver  halides 210 

determination  with  titration  of  the  cations 214 

separation  by  graded  potential 211 

Halogen  salts,  behavior  toward  the  current 48 

Hard  lead 301,  310 

Heat,  action  upon  cupric  salts 118 

influence  on  diffusion 118 

influence  on  separations  in  complex  electrolytes 93 

Heating  of  electrolytes 61, 93 

Heavy  metals,  separation  from  alkalies  and  alkaline  earths 218 

Hildebrand's  mercury  cathode ' 65 

Historical 101-115 

Bollard's  gauze  electrodes 60 

Hydrides,  formation  of 22,  53 

Hydrogen  discharge  during  electro-analysis 22,  48 

effect  of  rate  of  stirring 63 

facilitated    by    increasing    the    concentration    of 

hydrogen  ions 197,  282 

discharge  of  other  ions,  and  oxidation  .    282 

overvoltage 82,  89,  92,  161,  169, 174 

prevented  by  forming  of  complexes 176 

significance  for  metal  depositions 

22..  36,  53,  80,  81,  88,  148 

Hydrosulphite  of  sodium  for  reduction  of  polysulphides 155 

Hydroxylamine,  use  in  copper  determination 117 

I 

Inclusions 156,  199,  203,  207 

Indium,  rapid  deposition 179 

Intensity,  or  current  strength 7 

Iodine,  deposition  as  silver  iodide 210 

separation  from  bromine  and  chlorine 213 

lonization,  theory  of 4 

Ions 4 

charge  residing  on 9, 10, 11 

concentration '. .  .        6 

increase  and  diminution  of 86-89 

migration  of 13 

polyvalent 10 

symbols  of 10 

univalent 10 

Iridium,  separation  from  platinum .     253 


336  INDEX 

PAGE 

Iron,  analysis  of  commercial 310 

anodes 187 

deposition  from  oxalate  solution 181,  183 

deposition  with  aid  of  magnetic  stirring 76,  77 

deposits,  carbon  in 181 

electrolytic,  as  standard  in  volumetric  analysis 182 

ores 310 

rapid  deposition  of 184 

separation  from  alkalies  and  alkaline  earths 218,  220 

aluminium 269,  270 

aluminium  and  beryllium 272,  273 

aluminium  and  chromium 272 

aluminium,  uranium,  thorium,  lanthanum,  prase- 
odymium, neodymium,  cerium,  zirconium,  tita- 
nium and  phosphoric  acid 273,  274 

cadmium 261 

chromium . 270,  271 

chromium  and  uranium 272 

cobalt  and  nickel 265 

copper 121,  127,  237,  238*293 

lead 275,284 

manganese 267,  268 

mercury 251 

uranium 270 

vanadium 274 

zinc 266 

J 

Joule,  definition  of 13 

K 

Kiliani,  significance  of  voltage 23,  33 

Kilowatt,  definition  of 13 

Kohlrausch,  use  of  Wheatstone  bridge   19 

Kollock  and  Smith,  mercury  cathode , 65 

L 

Lactic  acid,  electrolytic  reduction  of 50 

Lanthanum,  separation  from  alkalies  and  alkaline  earths 219 

iron 274 

Lattice  stirrer 66,  68 

Lead 193,  284 

crude 310 

deposition  in  the  presence  of  copper 197,  232, 304 

determination  as  lead  dioxide 193 

determination  in  lead  sulphate 232 

dioxide,  composition  of 194,  195 

solution  of 195 

hard  301,  310 


INDEX  337 

PAGE 
Lead, peroxide  (see  lead  dioxide). 

rapid  deposition 196 

refined 307 

separation  from  antimony 284 

arsenic,  chlorine,  selenium,  manganese,  silver  and 

bismuth 196,  308,  310 

separation  from  cadmium 284 

copper 130,  230,  232 

iron 275 

nickel 275 

other  metals 284 

silver 246,  248 

zinc 259 

soft 307 

sulphate,  electro-analysis  of 230,  232 

solution  of 304 

tetranitrate 193 

tin-antimony-copper  alloys 300 

Lippmann's  capillary  electrometer 43 

Lithium,  separation  from  calcium 218,  219 

magnesium  and  the  heavy  metals 218 

uranium 221 

Logarithms ; 322 

M 

Maclnnes  and  Adler,  theory  of  over  voltage 82 

Magnesium  and  calcium,  separation  from  alkalies 218,  219 

Magnesium,  separation  from  alkalies  and  alkaline  earths 218 

barium  and  strontium 219 

cadmium 259 

calcium 221 

copper .          • 235,  239 

lead 284 

mercury 251 

Magnetic  stirring 73 

Manganese,  deposition  as  manganese  dioxide 197,  200,  201 

separation  from  alkalies  and  alkaline  earths 218 

cadmium 262 

copper 239 

iron 267,268 

lead 196 

mercury 251 

zinc 258 

dioxide,  composition  of 199 

peroxide  (see  dioxide) 

silicide 318 

Mansfeld  electrodes 54 

Marsh  test  with  aid  of  electric  current .  .  162 


338  INDEX 

PAGE 

Medway,  Gooch  and .  .  . 65 

Mercury,  behavior  in  the  capillary  electrometer 43 

cathode 59,  75,  80,  205,  214 

compounds,  insoluble 137,  318 

deposition  from  cyanide  solution 136 

nitric  acid  solution 135,  136 

sodium  sulphide  solution 137 

determination  in  cinnabar 318 

electromotive  force  of  the  drop  electrode 39 

separation  from  the  alkalies,  alkaline  earths  and  magnesium  .     251 

aluminium 250 

antimony,  arsenic  and  tin 250 

cadmium 251,  262 

cobalt,  nickel  and  iron 251 

copper 230 

lead 284 

manganese  and  selenium 251 

tellurium,  bismuth  and  zinc 252 

single  potential 4    40 

Metal  deposits,  nature  of 52 

Metals,  deposited  as  such 79 

oxides 79 

deposition  from  simple  and  from  complex  electrolytes 83 

Migration  of  the  ions 13 

rate  of 15 

Mole,  definition  of 26 

Molybdenite 208 

Molybdenum,  deposition  as  oxide . . 206 

separation  from  alkalies 207 

copper 241 

vanadium 286 

N 

Nature  of  deposits 52,  80,  86  87,  88 

NDioo * 124 

Neodynium,  separation  from  alkalies  and  alkaline  earths 219 

iron 274 

Nernst  formula 27,  84 

significance  in  electro-analysis 84,  89 

theory  of  electromotive  force 27 

Neumann's  potential  series  of  the  elements 26,  81 

Neutrality,  electrical '. 5 

Nickel-ammonia  cation .  185 

Nickel,  behavior  toward  potassium  cyanide 263 

carbon  content  of  deposits 190 

coin 241 

commercial 311 

deposition  from  ammoniacal  solution 185,  189 


INDEX  339 

PACE 

Nickel,  deposition  from  chloride  solutions  ..............  .  ...........     188 

nitrate  solutions  ...........................     186 

oxalate  solutions  ......................      190,  191 

determination  in  alloy  steels  ..........................       312,  314 

with  aid  of  magnetic  stirring  ...............................       77 

Nickel,  reaction  with  bromine  ....................................  "  188 

separation  from  *lk*lipg  and  alkaline  earths  ..................     218 

and  uranium  ...................  .    280 


cadmium  ...........................        2*i2 

cobalt  ....................................     281 

copper  .......................     120,  130,  240,  241 

chromium  .................................     279 

chromium,  aluminium  and  manganese  ........     190 

iron  ......................................     265 

lead  .................................      275,  284 

mercury  ..................................    251 

xinc  .................................       275,  278 

steel  ..................................................     312 

test  with  ammonium  sulphide,  uncertainty  of   ...............     188 

Nitric  acid,  addition  in  copper  determination  .......................     117 

behavior  toward  the  current  .......................     48,  117 

in  presence  of  copper  and  sulphuric  acid  .........       48 

determination  in  nitrates  .............................     *&& 

importance  in  metal  separations  .......................       49 

its  role  in  lead  dioxide  deposition  ..................       193,  197 

transformation  into  ammonia  ..................      48,  117,  127 

Nitrobenzene,  reduction  of  ........................................     49,  50 

Nitrogen,  determination  in  organic  substances  ......................     223 

indirect  determination  ..................................    221 

Non-electrolytic  methods  ............................................       97 

Normal  electrode  .........................  :  ......................       40 

element,  Weston  ..........................  ...............      37 

sulphuric  add  ..........................  .  ................     119 

O 
Oettel's  fork  electrode  ...........................................       60 

Ohm,  definition  of  ...............................................         8 

Ohm's  resistance  in  electrolytes  ..........................  .........       32 

......................  7,8,16,17 

applicable  to  electrolytes  ............................      29,32 

Organic  compounds  behavior  toward  the  current  ...................      49 

Osmotic  pressure  ............................................      23,  24 

in  the  Nernst  formula  ............................      84 

Overvoltage  of  hydrogen  on  different  metals  ..............     82,  89,  92,  174 

oxygen  at  the  anode  ...............................     118 

Oxalic  acid,  as  conductance  salt  ...................................      17 

electrolytic  reduction  of  .................................       50 

transformation  into  glycoDie  acid  ......................     190 


340  INDEX 

PAGE 

Oxalic  acid,  valence  of  carbon  in Ill 

Oxalates,  behavior  toward  the  current 50 

Oxidation,  definition  of , 4,  110 

Oxidation  potential 25,  26 

Oxides,  deposited  by  the  current 79 

Oxygen,  evolution  during  electrolysis 47 

injurious  effect  in  the  determination  of  halogens 214 

salts,  behavior  toward  the  current 47 

P 

Palladium,  rapid  deposition  of 61,  142 

separation  from  copper 242 

gold 253 

dissolving  the  deposit 143 

Passive  iron  as  anode 187 

Paweck's  gauze  electrodes 59 

Perkins'  gauze  electrodes 60 

Peroxides 145,  194 

Phosphates,  deposition  in  the  presence  of 116*192 

Phosphor-bronzes 300 

Phosphoric  acid,  determination  in  the  presence  of  sodium . 218 

separation  from  iron . . .     274 

Plating  platinum  electrodes  with  copper  and  silver 

141,  160,  161,  167,  170,  172, 175,  217 

Platinum,  attacked  by  potassium,  cyanide 137 

ammonia .      187,  188 

in  the  deposition  of  indium 179 

manganese  dioxide 198 

mercury 137 

zinc 173 

black 141 

deposition  of 141,  142 

dishes,  Classen's 57 

electrodes,  coated  with  copper,  cadmium  and  tin.  .  141,  160, 

161,  164  167,  170,  172,  175,  217 

platinized 82 

iridium  dishes 198 

separation  from  copper 242 

gold 252 

iridium 253 

silver 248 

Polarization 23 

current,  formation  of   30 

measurement  of    31,  35 

Polarization  potential,  least  value 32 

Polarity Ill 

Polysulphides,  reduction  by  potassium  cyanide,  etc. 155 


INDEX  341 

PAGE 

Potassium  argenticyanide,  dissociation  of 133 

chloride,  analysis  of  217 

cuprocyanide,  dissociation  of 51 

cyanide,  action  upon  platinum 137 

for  reduction  of  polysulphides 155 

determination  in  the  presence  of  anions 217 

indirect  determination 221 

Potassium  mercurocyanide,  dissociation  of 136 

separation  from  aluminium  and  iron 220 

calcium,  magnesium  and  heavy  metals 219 

uranium 221 

Potential 21 

between  the  electrodes 34,  147 

control  of,  in  the  bismuth  determination 147 

decomposition 31 

deposition  with  constant 118 

difference,  formation  of 24,  25 

source  of  the  current 27 

measurement  of 37 

drop 35,  38 

at  the  anode  independent  from  that  at  the  cathode 36 

of  a  metal  against  the  solution 25 

separation  by  graded 80 

series  of  the  metals 26,  81 

significance  for  electro-analysis 84,  86 

single 26 

measurement  of 37,  66 

of  value  zero 38 

of  the  noble  and  base  metals 26,  81 

Praseodymium,  separation  from  alkalies  and  alkaline  earths 219 

iron 274 

Q 

Quantitative  deposition  by  the  current 89,  90 

Quantity  of  electricity,  unit  of   11 

R 

Radium,  emanations  from 108 

Rapid  deposition  of  cadmium  in  potassium  cyanide  solution 177 

copper  in  nitric  acid  solution 128 

sulphuric  acid  solution 121 

gold  in  potassium  cyanide  solution    139 

indium  in  formic  acid  solution 179 

iron  in  oxalate  solution 184 

lead  in  nitric  acid  solution 196 

manganese  in  acetate  solution 200 

mercury  in  nitric  acid  solution 136 


342  INDEX 

PAGE 

Rapid  deposition  of  nickel  in  ammoniacal  solution 189 

oxalate  solution 191 

platinum  in  sulphuric  acid  solution 142 

tellurium  in  tartrate  solution .  .  .  .' 163 

electrolytic  work 60,  73 

electrodes  suitable  for 64-67 

outfit  at  Aachen 64,  68,  70 

proposed  by  Fairlie  and  Bone 73 

Rapid  oxidation  of  chromium  to  chromate 205 

separation  of  cadmium  from  aluminium 259 

iron 261 

copper  from  alkalies,   aluminium,   magnesium   and 

alkaline  earths 235 

arsenic 234 

cadmium 229 

iron 238 

lead 232 

mercury 230 

nickel 240 

platinum 242 

silver 227 

zinc 243 

gold  from  palladium 253 

platinum 252 

iron  from  aluminium 269 

aluminium,    uranium,   rare   earths,    tita- 
nium and  phosphorus 273 

chromium 271 

mercury  from  bismuth 252 

nickel  from  chromium 279 

zinc 278 

silver  from  aluminium 244 

bismuth 248 

lead 248 

zinc 250 

Rate  of  stirring  and  duration  of  electrolysis 61,  62 

Reaction  resistance  in  potassium  cyanide  solutions 95 

Reduction,  definition  of 4,  110 

Refined  lead 307 

Resistance 7,  16 

measurement  of 18 

Ohm's,  in  liquids,  measurement 32,  33 

unit  of 7 

specific 20 

Rhodium,  rapid  deposition  of    144 

Rotating  electrodes 57,  58,  66,  67 

Rothe's  method  for  removing  iron 313 

Roughened  platinum  dishes 59,  156,  194 


INDEX  343 

S 

PAGE 

Sand's  auxiliary  electrodes 40,  42,  147 

electrodes 65,  66,  67 

Screw-shaped  stirrer 66,  68 

Selenium,  separation  from  copper 242 

lead 196 

silver 251 

Separation,  in  general 33,  36 

in  simple  and  complex  electrolytes 

45,  50,  79,  80,  146,  225,  226,  245,  263 

Sieve  anode .  .  57 

Silver  anodes,  regeneration  of    211,  213,  215 

compounds,  insoluble 134 

decomposition  potential  in  cyanide  solutions 94 

deposition  from  ammoniacal  solution 133 

potassium  cyanide  solutions 

87,  88,  95,  133,  134 

nitrate  solutions 131 

peroxide,  reduction  by  alcohol 131 

separation  from  aluminium „. 244 

antimony 245 

arsenic 246 

bismuth 248 

cadmium 263 

copper 225,  227 

lead 196,  246,  248 

platinum 248 

selenium 249 

zinc 33,  249,  250 

Simple  electrolytes,  behavior  toward  the  current 45 

Single  potential  of  value  zero 38,  39 

potentials,  independent  of  one  another 25 

measurement  of 35,  37 

of  noble  and  base  metals 81,  91 

values 26 

Sliding  contact 38,  43,  149 

Solenoid  method  of  Heath  123 

Smith  and  Kollock,  mercury  cathode 65 

Sodium,  chloride  solution  electrolysis  of 48 

determination  in  the  presence  of  carbonate  and  phosphate ....    218 

hydrosulphite  for  reduction  of  polysulphides 155 

separation,  from  aluminium,  iron  and  uranium 220,  221 

calcium,  magnesium  and  heavy  metals 219 

sulphate,  complex  in  the  presence  of  sulphuric  acid 176 

sulphide,  pure 158 

sulphite,  for  reduction  of  polysulphides 155 

Soft  lead 307 

Solder ,    300 


344  INDEX 

PAGE 

Solution  pressure 23 

electrolytic 24 

in  the  Nernst  formula 27,  84 

measurement  of 85 

Spear  and  Strahan,  determination  of  zinc 167 

Special  analyses 287 

Specific  conductance 20 

resistance 20 

Sphalerite 306 

Spongy  deposits,  formation  of 23,  53,  88,  131,  148 

prevention  in  copper  determination ...: 117 

Steel,  analysis  of 310,  311,  312 

Stirred  electrolytes 53,  60,  73 

Stirrer  for  rapid  electrolytic  work 64,  66,  68,  71 

Stirring,  by  electro-magnetic  effect 73 

effect  in  simple  and  in  complex  electrolytes 61-64 

Storage  cells 70,  120,  130,  145 

Strontium  bromide,  determination  of  both  constituents  in 220 

deposition  as  amalgam 209 

separation  by  graded  potential 209 

from  aluminium,  calcium,  magnesium  and  iron 

218-220 

cadmium 259 

copper 235 

lead    '.     284 

mercury 251 

uranium 221 

Succinic  acid,  behavior  toward  the  electric  current 50 

Sulpho  salts,  complex 153 

transformation  into  oxalates 160 

Sulphuric  acid,  behavior  toward  the  current 29,  47 

conductance 20,  29,  143 

standard  solution  of 224 

Surface  tension  of  mercury 43 

Suspensions , 59,  238,  266,  290,  294,  306,  312 

T 

Tafel,  J,  overvoltage  values 92 

Tartrates,  carbon  deposits  from .  T 182 

Tellurium,  deposition  from  tartrate  solutions 163 

separation  from  copper 242 

mercury 252 

Temperature,  action  upon  cupric  salts 118 

effect  on  diffusion 118 

influence  on  separations  in  complex  electrolytes 93 

Tension,  electrolytic  solution 23,  24 

Test  cathode  . .  305 


INDEX  345 

PAGE 

Thallium,  deposition  as  metal 202 

oxide 202 

Thermopiles  for  constant  voltage 131,  212 

Thorium,  separation  from  alkalies  and  alkaline  earths 219 

iron 274 

Thiosalts,  complex 153 

transformation  into  oxalates 160 

Tin,  commercial 314 

deposition  from  ammonium  sulphide  solutions 78, 161 

oxalate  solutions 159 

foil.  . . 300 

lead-antimony-copper  alloys 301,  302 

separation  from  alkalies  and  alkaline  earths 219 

antimony 253,  255,  301 

antimony  and  arsenic 256 

copper 244 

mercury 250 

Titanium,  separation -from  alkalies  and  alkaline  earths 219 

iron i . 274 

Titration  of  alkalies  and  alkaline  earths  after  deposition  as  amalgams .  .     219 

Treadwell  and  v.  Girsewald,  complexity  of  copper  cyanide 96 

True  potentials 82,  92 

Tungsten,  separation  from  copper 242 

Type  metal c 301 

U 

Uranium,  deposition  as  oxide 201 

separation  from  alkalies,  alkaline  earths 219,  221 

aluminium  and  nickel 280 

cobalt 281 

copper , 243 

iron 270,  274 

iron  and  chromium 272 

lead 284 

Urea,  use  in  the  copper  determination 117 

cadmium  determination ....  . , 179 

Units,  electrochemical  (Faraday) 11 

of  conductance % 20 

current  strength k 7,  8 

electromotive  force t 8 

quantity  of  electricity 11 

resistance , k 7 

V 

Vanadium 208 

separation  from  alkalies  and  alkaline  earths 219 

iron 274 

molybdenum 286 


346  INDEX 

PAGE 
Valence  of  the  elements,  influence  on  electrolysis  ...  9,  130,  157,  234,  254,  256 

Voltage 22 

deposition  with  constant.  ...      118,  119,  130,  131,  145,  207,  229,  240 

measurement  of 34,  35 

separations  by  graded 80 

Volt,  definition  of 8 

Voltmeter 34,  35 

W 

Washing  electrodes  after  breaking  the  circuit 120 

while  stirring . . 152 

without  interrupting  the  current 120,  126,  175 

Water,  decomposition  of 47,  112 

Watt,  definition  of  10 

Wave-shaped  anode 65 

Weston's  normal  element 37 

Wheatstone  bridge 19,  37 

White  metals : 300,  301,  302 

Winkler's  gauze  electrodes * 59 

\ 

Z 

Zero  instrument 43,  149 

Zinc  chloride,  commercial 303 

decomposition  potential  in  potassium  cyanide  solutions 95 

deposition  from  acetate  solutions 169,  172 

alkaline  solutions 165 

alkaline  tartrate  solutions 279 

ammoniacal  solutions 168 

oxalic,  tartaric  and  formic  acid  solutions 172 

dust 305 

ores 305 

separation  from  alkalies  and  alkaline  earths 219 

aluminium,  lead  and  bismuth    258,  259 

cadmium 81,  264 

cobalt 280 

copper 78,  130,  243,  296 

iron 266 

lead    284 

manganese 258 

mercury 252 

nickel   275,  278 

silver    33,  250 

Zirconium,  separation  from  alkalies  and  alkaline  earths 219 

iron 274 

lead. .  284 


PLATE  I. 


PLATE  II. 


0.5  A. 

Fuse 


.    Motor 


lOA.KFuse      m         10  A.   d     10  A.  td  Fuse        „        10A.*LFuse  10  A. 


L   12      13   14   15   16         17   18   19   20      21   22   23   24 


Wiley  Special  Subject  Catalogues 

For  convenience  a  list  of  the  Wiley  Special  Subject  Catalogues, 
envelope  size,  has  been  printed.  These  are  arranged  in  groups 
— each  catalogue  having  a  key  symbol.  (See  Special  Subject 
List  Below).  To  obtain  any  of  these  catalogues,  send  a 
postal  using  the  key  symbols  of  the  Catalogues  desired. 


1 — Agriculture.     Animal  Husbandry.     Dairying.     Industrial 
Canning  and  Preserving. 

2 — Architecture.       Building.      Masonry. 

3 — Business  Administration  and  Management.     Law. 

Industrial  Processes:   Canning  and  Preserving;    Oil  and  Gas 
Production;  Paint;  Printing;  Sugar  Manufacture;  Textile. 

CHEMISTRY 
4a  General;  Analytical,  Qualitative  and  Quantitative;  Inorganic; 

Organic. 
4b  Electro-  and  Physical;  Food  and  Water;  Industrial;  Medical 

and  Pharmaceutical;  Sugar. 

CIVIL  ENGINEERING 

5a  Unclassified  and  Structural  Engineering. 

5b  Materials  and  Mechanics  of  Construction,  including;  Cement 
and  Concrete;  Excavation  and  Earthwork;  Foundations; 
Masonry. 

5c   Railroads;  Surveying. 

5d  Dams;  Hydraulic  Engineering;  Pumping  and  Hydraulics;  Irri- 
gation Engineering;  River  and  Harbor  Engineering;  Water 

Supply. 

(Over) 


CIVIL  ENGINEERING—  Continued 

5e  Highways;  Municipal  Engineering;  Sanitary  Engineering; 
Water  Supply.  Forestry.  Horticulture,  Botany  and 
Landscape  Gardening. 


6 — Design.  Decoration.  Drawing:  General;  Descriptive 
Geometry;  Kinematics;  Mechanical. 

ELECTRICAL  ENGINEERING— PHYSICS 

7 — General  and  Unclassified;  Batteries;  Central  Station  Practice; 
Distribution  and  Transmission;  Dynamo-Electro  Machinery; 
Electro-Chemistry  and  Metallurgy;  Measuring  Instruments 
and  Miscellaneous  Apparatus. 


8 — Astronomy.      Meteorology.      Explosives.      Marine    and 
Naval  Engineering.     Military.     Miscellaneous  Books. 

MATHEMATICS 

9 — General;    Algebra;   Analytic  and   Plane   Geometry;    Calculus; 
Trigonometry;  Vector  Analysis. 

MECHANICAL  ENGINEERING 

lOa  General  and  Unclassified;  Foundry  Practice;  Shop  Practice. 
lOb  Gas  Power  and    Internal   Combustion  Engines;  Heating  and 

Ventilation;  Refrigeration. 
lOc  Machine  Design  and  Mechanism;  Power  Transmission;  Steam 

Power  and  Power  Plants;  Thermodynamics  and  Heat  Power. 

11 — Mechanics.  

12 — Medicine.  Pharmacy.  Medical  and  Pharmaceutical  Chem- 
istry. Sanitary  Science  and  Engineering.  Bacteriology  and 

Biology. 

MINING  ENGINEERING 

13 — General;  Assaying;  Excavation,  Earthwork,  Tunneling,  Etc.; 
Explosives;  Geology;  Metallurgy;  Mineralogy;  Prospecting; 
Ventilation. 


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